president's address. 31 



very important aucl deteruiiaiug principle in all that relates to heiedity 

 and variation. It remains, however, to be shown how far the Mendelian 

 phenomenon is general. And it is, of course, admitted on all sides that, 

 even were the Mendelian phenomenon general and raised to the ranlc of a 

 law of heredity, it would not be subversive of Mr. Darwin's generalisations, 

 but probably tend to the more ready application of them to the explana- 

 tion of many difficult cases of the structure and distribution of organisms. 



Two general principles which Mr. Darwin fully recognised appear to 

 me to deserve more consideration and more general application to the 

 history of species than he had time to give to them, or than his followers 

 have accorded to them. The first is the great principle of ' correlation of 

 variation,' from which it follows that, whilst natural selection may be 

 favouring some small and obscure change in an unseen group of cells — 

 such as digestive, pigmentary or nervous cells, and that change a change 

 of selective value — there may be, indeed often is, as we know, a correlated 

 or accompanying change in a physiologically related part of far greater 

 magnitude and prominence to the eye of the human onlooker. This 

 accompanying or correlated character has no selective value, is not au 

 adaptation — is, in fact, a necessary but useless by-product. A list of a 

 few cases of this kind was given by Darwin, but it is most desirable that 

 more should be established. For they enable us to understand hov/ it is 

 that specific characters, those seen and noted on the surface by systema- 

 tists, are not in most cases adaptations of selective value. They also open 

 a wide vista of incipient and useless developments which may suddenly, 

 in their turn, be seized upon by ever-watchful natural selection and raised 

 to a high pitch of growth and function. 



The second, somewhat but by no means altogether neglected, principle 

 is that a good deal of the important variation in both plants and animals 

 is not the variation of a minute part or confined to one organ, but has 

 really an inner physiological basis, and may be a variation of a whole 

 organic system or of a whole tissue expressing itself at several points and 

 in several shapes. In fact, we should perhaps more generally conceive 

 of variation as not so much the accomplishment and presentation of 

 one little mark or difierence in weight, length, or colour, as the 

 expression of a tendency to vary in a given tissue or organ in a particular 

 Avay. Thus we are prepared for the rapid extension and dominance of 

 the variation if once it is favoured by selective breeding. It seems to 

 me that such cases as the complete disappearance of scales from the integu- 

 ment of some osseous fishes, or the possible retention of three or four scales 

 out of some hundreds present in nearly allied forms, favour this mode of 

 conceiving of variation. So also does the marked tendency to produce 

 membranous expansions of the integument in the bats, not only between 

 the digits and from the axilla, but from the ears and diSerent regions of the 

 face. Of course, the alternative hairy or smooth condition of the integu- 

 ments both in plants and animals is a familiar instance in which 

 a tendency extending over a large area is recognised as that which 



