SECTIONAL TRANSACTIONS.— K. 399 



Monday,. September 11. 



14. Joint Discussion with Section D on The Present I'osiliun of 

 harwinism. 



(a) Dr. J. 0. Willis, F.R.S. — The Inadequacy of the Theory of 

 Natural Selection as an Explanation of the Facts of Geographical 

 Distribution and Evolution. 



Darwin's immortal service to science consists in the permanent establisliment 

 of the doctrine of evolution. This he effected by devising the simple and 

 beautiful mechanism of natural selection of infinitesimal variations— the 

 principle usually known under the name of Darwinism. 



This doctrine makes several assumptions : among others, that variation re- 

 sulting in evolution is (1) continuous, (2) hereditary, (3) differentiating, 

 (4) selected, and (5) that the necessary variations appear. For all of these 

 the proof is as yet insufficient. 



Work carried on during the last thirty years has finally led to the demon- 

 stration that the phenomena both of evolution and of geographical distribution 

 may be represented by hollow curves, which are always of the same type, which 

 are closely parallel in both plants and animals, and which may be found in 

 thousands of cases. Taken in averages of tens of allied forms, area occupied 

 goes with age of species and with size of genera. In other words, size and 

 area show the same phenomena, both increasing with age. Natural selection, 

 a differentiating cause, could not produce such uniformity of expression. 

 Evolution would appear to have unfolded itself with time upon a dominant 

 plan, natural selection simply acting as an agent destroying the unfavourable 

 (and probably many other) variations. 



(h) Mr. G. Udnt Yule, F.E.S. — 4 Mathematical Conception of 

 Evolution based on the Theory of Age, Size, and Space. 



If, comparing allied groups, the more widespread species are on the whole 

 the older, and the larger genera on the whole the most widespread, it follows 

 that the size of the genus must on the whole be an index of its age. This 

 suggests that species may be regarded as thrown by the genus much as offspring 

 are thrown by a stock, and that the number of species originating from a 

 given initial species will increase in geometric ratio with the time. 



The forms of frequency distribution for numbers of genera with given 

 numbers of species to which this conception leads are shown to be in accordance 

 with the facts, and the possibility is suggested of determining from such 

 distributions the ratio between the rates of increase of genera and species and 

 the age of the family in terms of the doubling period for species. 



(c) Mr. C. Tate Eegan, F.R.S. 



{d) Prof. W. JOHANNSEN. 



(e) Mr. J. T. Cunningham. — Origin of Species and Origin of 

 Adaptations. 

 Darwin's theory of Natural Selection is based on the assumption that the 

 species and its diagnostic characters, either visible or invisible, are adapted to 

 some special habits or conditions peculiar to each species ; in other words that 

 the characters, or the successive steps in the evolution of characters, have been 

 selected. Evidence in the vast m.ajority of cases is against this assumption and 

 in favour of the conclusion that diagnostic characters have nothing to do with 

 adaptation. It is now known that many species spontaneously give off mutations, 

 and that those are not due to influence of conditions in experiment or cultiva- 

 tion. They occur in Nature, and some of them may be incapable of survival in 

 Nature, but this does not imply that those which can survive are due to selection. 

 So far I am in agreement with Jlr. Udny Yule and Dr. Willis. But I do not 

 understand what is meant by the statement that the area occupied goes with age 

 of species. How are we to know which species is the older? It may be 

 legitimate to infer age from area occupied, but not to assume age except from 

 morphological evidence. Man is cosmopolitan in distribution and is one of the 



