NA TURE 



197 



THURSDAY, DECEMBER 27, 1900. 



A CONTRIBUTION TO LAMARCKIAN 

 EVOLUTION 

 Sexual Dimorphism in the Animal Kingdom ; a Theory 

 of the Evolution of Secondary Sexual Characters. By 

 J. T. Cunningham, M.A. Pp. xi + 317 ; with 32 

 illustrations. (London : Adam and Charles Black, 

 1900.) 



HOWEVER much readers of this work may dissent 

 from the views of the author, there can be no doubt 

 that the volume is worthy of the most careful perusal. 

 For the first time since the publication of Darwin's 

 theory of sexual selection we have been provided with a 

 bold and intelligible attempt at explaining secondary 

 sexual characters on Lamarckian principles, and although 

 many of us may arrive at the conclusion that Mr. 

 Cunningham has not succeeded in establishing his case, 

 it will be generally admitted that he has discussed the 

 problem, on the whole, in a more or less scientific spirit, 

 and has supported his arguments by a body of well-con- 

 sidered and, in many cases, original observations, which 

 make his book exceptionally valuable as a storehouse of 

 facts. 



The author, as is well known, belongs to that school of 

 anti-Darwinian evolutionists which accepts the broad 

 doctrine of descent with modification, but which denies 

 the sufficiency of natural selection as the cause of species 

 formation*. In the introduction he re-states some of the 

 chief difficulties and objections which have been urged, 

 over and over again, by the opponents of Darwinism. 

 An analysis of these objections, as set forth by Mr. 

 Cunningham, will show that they resolve themselves 

 mainly into the inutility of incipient stages, the dictum of 

 Romanes that natural selection is a theory of the origin 

 of adaptations, the inutility of specific characters, the 

 failure of natural selection to account for the origin of 

 variation and so forth. Students of evolution are so 

 familiar with these much-discussed topics that we may 

 be excused from dealing with them again in detail. It 

 is ancient history that Darwin admitted " use and 

 •disuse " and the " direct action of external conditions " as 

 factors of some value in the production of species. But 

 he assigned a subordinate function to these agencies, 

 and it is quite unfair to Darwin's position to state, as 

 Mr. Cunningham does (p. 12), that "if once we admit 

 this, selection becomes a secondary and subordinate 

 character." 



What concerns us most here, however, is not so much 

 the destructive part of the present work, because the 

 author, in brilliant contrast to many critics belonging to 

 his school, has not contented himself with mere cavil or 

 with the watering-down process which is rife among cer- 

 tain sections of naturalists who regard with horror any 

 attempt at dealing with the species problem by scientific 

 method. Mr. Cunningham has formulated his own views, 

 and has applied them to the particular, and, we may add, 

 absorbingly interesting, class of phenomena presented by 

 animals with dissimilar sexes. These views are, as the 

 author will admit, purely Lamarckian — in fact we might 

 say grossly mechanical, since the secondary sexual 

 NO. 1626, VOL. 63] 



characters are regarded as the direct result of mechanical 

 irritation or stimulation (p. 37). It is needless to point 

 out that this view is absolutely at variance with that held 

 by selectionists. It is a doctrine which has been 

 broached of late years to account for floral structures, 

 and which, if we are not mistaken, has received but little 

 favour from botanists. 



In defining Mr. Cunningham's position as a Lamarckian, 

 it is necessary to point out, in order that full consideration 

 may be given to his views, that he has introduced a cer- 

 tain modification into that doctrine which he claims — and 

 we think rightly — to be original. Lamarckism, of course, 

 carries with it the admission that acquired characters are 

 hereditary, and the author's attitude towards this question 

 will be considered subsequently. But whereas the 

 original, or proto- Lamarckian, or his modern successor, 

 the neo- Lamarckian, never appears to have troubled him- 

 self very much about the precise period in the life of the 

 individual at which the " acquired " characters were pro- 

 duced, Mr. Cunningham has laid it down, as the essential 

 part of his amendment, that these characters only become 

 developed (by heredity) at " that period of life and in 

 that class of individuals in which they were originally 

 acquired " (p. yj). He further postulates that, in order to 

 produce such hereditary acquired characters, the stimu- 

 lations must "be regularly recurrent," and their trans- 

 mitted effect is then only developed " in association with 

 the physiological conditions under which they were 

 originally produced" (p. 41). In other words, he re- 

 states Darwin's " inheritance at corresponding periods of 

 life " and " inheritance as limited by sex " from the 

 Lamarckian platform, and imposes a new restriction in 

 the way of "physiological conditions" which are nowhere 

 defined throughout the work excepting in the case of 

 secondary sexual characters, where the supposed con- 

 ditions are vaguely associated with the change of 

 constitution accompanying sexual maturity. 



From these considerations it follows that the external 

 stimulus or irritation which (admittedly) can modify a 

 part or organ of an individual during its lifetime, is only 

 capable of producing modifications of specific rank when 

 applied continuously, throughout many generations, at 

 some particular, and at present undetermined, state of 

 physiological activity. Supposing we admit, for the 

 moment, that the author's position is sound for the only 

 case in which such special physiological conditions are 

 hinted at, viz., the period of puberty, then it follows that 

 those mutilations which have been carried out through 

 successive generations in many tribes of savages at the 

 precisely critical period required by the hypothesis might, 

 at any rate, be expected to show now and again a tendency 

 to appear spontaneously in the offspring at that period. 

 The evidence on this point is certainly against the author, 

 and the case for Lamarckism in its amended form re- 

 ceives no more support by virtue of Mr. Cunningham's 

 amendment than did the original Lamarckism from the 

 consideration of such classes of facts. It is, perhaps, not 

 going too far to say that the author's position is less ten- 

 able by virtue of his own restriction than that of the older 

 Lamarckians, because the whole explanation of sexual 

 dimorphism, from Mr. Cunningham's standpoint, is made 

 to depend upon the action of external stimuli applied at 

 the period of breeding, i.e. at sexual maturity. 



K 



