Hogben &apos;s paper is thus of some value as a counsel of scientific caution , but adduces no fundamental objections to the theory . in the preceding pages an attempt has been made to clarify the issues and gain some idea of the influence of genetic factors in one aspect of language , the sound structure . this has been taken broadly , and the sound-producing apparatus and its results , whether in the individual , the group , or the population , the complement of sounds of a language and of the languages of a population , and the changes in a sound complement , have all been considered . each of these should have a genetic component . the development , structure and functioning of the vocal apparatus are clearly determined in part by genes , and hence the nature and the limits of the continuum of sound production possible to this apparatus must be likewise . and following from this , in the last analysis , any vocal sound produced by an individual , whether in speech or not , and if in speech , whether significant or not , will be of the nature that it is , in part at any rate , because of the particular genetic composition of that individual . similarly the complement of sounds used by a group in the vocalisation of its language and the total complement of sounds used by the various groups of a population in the vocalisation of their languages will be of their characteristic natures , in part , because of the particular genetic composition of that group or population . and it would seem that genetic factors must play some part in changes in a sound complement . this is not only in the sense that the intermediate stages in any case of sound change must have some genetic component , but also in the sense that the motivations which induce a community of speakers to make the change derive from those speakers , and hence axiomatically reflect , to some extent , their genetic composition . the most important of these motivations has been taken to be the tendency to economy of effort , a tendency which is known to be operative in a wide range of human activities , and which itself must be largely genetic in its determination - there is after all no difficulty in imagining its evolution in a species under conditions of natural selection . but even those changes which seem to be mainly the result of cultural influences of one sort or another will have a genetic component . the speakers of a dialect borrow , imitate , or learn sounds from other dialects , partly , perhaps , as I have suggested above ( p 205 ) in accordance with their own preferences , but mainly , it is usually assumed , because of their impulses to conform with what seems a desirable norm . such impulses will in theory also have a genetic component . they vary , as is common knowledge , from individual to individual and they doubtless vary also , in terms of mean values , from group to group . the result of this investigation has been to develop in more detail the hypothesis of genetic influence in the sounds of language , particularly with regard to the extent of its field of operation and to the nature of the way in which this influence is exerted . for the reasons given , it seems that the existence of a genetic component of language as such is a priori to be accepted ; the question which remains then is whether the further hypothesis of the extent and the nature of the genetic influence such as has been outlined in the preceding pages is valid . the answer to this must primarily depend on the success with which it is considered that this hypothesis may be applied to and shed new light on the observed data of the subject , and suggest further constructive work . there can be no doubt of its applicability to a considerable amount of the material in linguistics and its ancillary disciplines . it can be applied to the individual and to the particular rate , method , and accuracy of his acquisition of his sound complement , to the uniqueness of this , and to its aberration from the group mean . it can be applied to the group , to the mean of the group rate and method of acquisition , to the uniqueness of the group sound complement , and to the relations between overlapping groups and their dialects . further , it can be applied to the population , to the uniqueness of the total sound complement of a population , to the widespread similarities in the sound complements of its various constituent groups , and to the particular distributions of the sounds and sound features of that total sound complement . it also provides a basic factor in the causation of phonetic change , clarifying the nature and the role of the tendency to economy of effort in this phenomenon , and offers an explanation of many of its observed characteristics , including some , such as the parallel developments in related languages spoken by related peoples , or the long continued drifts in a sound complement , which have been peculiarly resistant to explanation in the past . and it suggests a number of lines of investigation which should be fruitful in the further development of the subject . some of the most obvious of these may now be considered briefly in turn . geographical distribution of sound features . the most obvious and important linguistically is the geographical distribution of sounds and sound features . the cases discussed in this book have been discovered largely by trial and error , and an adequate series of maps of the distributions of the main types of articulations is a prime desideratum . besides articulations in the strict sense , it would seem likely that there may be much of interest in the distribution of such features as type of syllabic structure , liaison and juncture phenomena , the restrictions on occurrence of specific sounds and sound types - practically all the languages of Europe from Dutch to Russian , for example , permit no voiced plosive in word final position - and so on . and further , among less detailed phenomena , the establishment of the distribution of languages with lexical tone would seem valuable in this connexion . at the moment we have very little idea of the distributions of such features , and the lines of research which they may suggest . the historical development of such distributions opens a new field in that the appearance of a specific sound type in one language need not be an event which is solely the result of conditions internal to that language ; it may be related to the occurrence of sounds of similar type in other languages , of the same or different family , in the same region . I have suggested in a previous article ( Brosnahan , 1959 ) , for instance , that the development of affricate articulations in the old high German consonant shift is part of a larger , but geographically limited phenomenon , a remarkable development of affricates over the last two thousand years and centred in the area now occupied by the western Slavonic languages , Hungarian and Albanian . our understanding of language and the deeper-lying factors in its development is likely to be very greatly extended by investigation along these lines . a further possibility is the comparison and mapping of acoustic features or characteristics of representative samples of different languages . with modern methods of recording , it is not difficult to devise techniques to determine , say , the mean distribution of energy over the range of frequencies used in speech . such mean distributions should vary from language to language with differences in their sound complements , and with differences in the relative frequency of occurrence of specific types of articulation . the mapping of such distributions may also be informative in bringing to light unexpected correspondences at the sound level among different languages . the mechanism of hearing . this leads to another field of investigation . the discussion of the vocal apparatus in this work has been confined to that of the sound-producing mechanism . but it is not unreasonable to expect that the capacities of the sound reception mechanism may also have exerted some influence on the development of the sound aspect of language . it is true there is little evidence that the auditory distinctiveness of specific sounds has much effect on their selection in a sound complement ( p 12 f above ) but other possibilities exist . the capacity of the ear and its mean sensitivity to different ranges of acoustic frequency are likely to vary from group to group and population to population of the earth &apos;s surface in accordance with differences in the genetic composition of the peoples involved , and it is not impossible that the general or average set of a language in the frequency scale shows some sort of correlation with this capacity . a case that springs to mind in this connexion is that of languages with lexical tone , and an investigation into audiograms of speakers of these languages and comparison with those of speakers of other types may be of interest . some possible indications of a close connexion between vocal language and the hearing mechanism have been found . the mechanical resolving power of the cochlea with regard to frequency , measured as the extent of the shift of the point of maximum response along the cochlear partition for a given ratio of frequency change , is practically independent of frequency in most animals . in the human ear , however , this resolving power is relatively low up to about 300 cycles per second and then shows an abrupt increase , reaching a relatively high value by 1,000 cycles per second ( von B&amp;eacute;k&amp;eacute;sy and Rosenblith , 1951 ) . since the range of frequencies most important for intelligibility seems to be that above 1,000 cycles per second , it is tempting to regard the human variation from the pattern in other animals as the result , at least in part , of adaptation in the course of evolution to these important frequencies of human vocal communication . if language or its forerunners has exerted such influence on the phylogenetic development of the hearing mechanism , it is not unlikely that this mechanism has also exerted some influence on the frequencies of language . the acquisition of speech . a field in which the influence of genetic factors is likely to be more easily recognised is that of the acquisition of a sound complement in the process of learning to speak . a real need is more work of the nature of that done by Irwin and his associates covering adequate numbers of children and carried out by investigators with standardised techniques , to determine in detail the norms of this acquisition in other communities and with other languages . besides their importance in demonstrating the influence of the heredity of the group , such norms would be of considerable value in pediatrics and child development generally as well as in speech therapy . they may be expected to vary , not only on the grounds of genetic theory , but also in accordance with our existing knowledge of child development : thus , for example , the recent investigations by Geber and Dean ( 1957 ) have indicated that the general development of young east African children is some months ahead of that of European children of corresponding ages . another field here , which would seem to be very important , but which as far as I know has hardly been touched , is the environment in which the child acquires its sound complement . though some information is available as to the nature and frequency of the sounds which the child produces , no attempt seems to have been made to determine the nature and the frequency of the sounds which the child hears at this period of its life . difficult though such research may be to plan and execute , it should not be neglected . it may well be that some correlation will be found between the nature of the stimulus from the environment and the nature of the child &apos;s development , and this must be considered in assessing the role of the genetic component in the process . investigation of this topic may also be of value by giving precision to the conception of the representative sample of the sound complement , which , it was suggested above ( p 140 ) , could be taken as the actual norm of the group in the experience of the individual . 