124 



SCIENCE. 



[Vol. XII. No. 29; 



as a fundamental factor in evolution, may be more intelligently 

 considered under some of its subordinate phases, as heredity, physi- 

 ological selection, sexual selection, primogenital selection, sexual 

 differentiation including philoprogeneity, hybridity, etc. 



Heredity, as expounded by the ablest biologists and as exempli- 

 fied in life, is a puissant factor in evolution, and, though essentially 

 conservative, must, through the marvellous power of atavism, tend 

 to increase individual variability. 



Physiological selection, as suggested by Mr. Catchpool and as 

 expounded by Romanes, is undoubtedly an important factor in 

 evolution. Romanes believes that wherever there has been modi- 

 fication of the reproductive organs introducing incompatibility be- 

 tween two individuals, even where there has been no other change 

 or variation, we have a valid cause of differentiation which in its 

 consequences must be important. Compatibility or fertility be- 

 tween individuals is of the very essence of selection. Natural 

 selection implies that this sexual divergence is subsequent to or 

 coincident with divergences in other directions ; physiological selec- 

 tion, that it antecedes them. This theory implies variation in the 

 reproductive organs, or departure from the parental type, in at 

 least two individuals of opposite sex simultaneously ; and with this 

 admission, for which we are justified in facts, physiological selec- 

 tion will preserve many peculiarities which need have no necessary 

 connection with the exigencies of life. 



Sexual selection may be said to act in two ways, — by conflict of 

 the males for possession of the female, or by attractiveness ; the 

 former being most conspicuous among mammals, the latter among 

 birds, and both coming conspicuously into play among insects. It 

 is rather difficult to define the limit of sexual selection as a factor 

 in evolution ; but I would not confound it with another factor, not 

 hitherto generally recognized, but which I think must be all-pow- 

 erful, namely, sexual differentiation. 



It seems evident that the mere differentiation of sex in itself has 

 been an important element in variation. This principle elaborated 

 by Brooks as a modification of the theory of pangenesis is a good 

 one, and in the main the male may be said to be the more complex 

 and to represent the progressive, and the female the more simple 

 and to represent the conservative, element in nature. When the 

 conditions of life are favorable, the female preponderates, and ex- 

 ercises a conservative influence. When the conditions are unfavor- 

 able, the males preponderate, and, with their greater tendency to 

 vary, induce greater plasticity in the species, and hence greater 

 power of adaptation. Sexual differentiation may, I think, be used 

 to include many other variations and differentiations not otherwise 

 satisfactorily accounted for, and to express the law of the interac- 

 tion of the sexes upon one another, inducing great differentiation 

 entirely apart from the struggle of the males for the possession of 

 the females, or the struggle for existence. 



Last of all I mention hybridity, which has been fully discussed 

 by many, and by no one more ably than by Darwin 'himself. 



Among the psychical conditions, the use and disuse of an organ 

 and its effect upon the offspring of the individual is of prime im- 

 portance. That functionally produced modifications are inherited 

 was the great assumption upon which Lamarck founded his theory 

 of evolution. Many able naturalists have insisted on it, and in my 

 judgment there should no longer be any doubt whatever of the 

 fact. The influence of emotion on the individual is closely con- 

 nected with this category', as strong mental effort may be made to 

 affect special parts of the body. 



An interesting problem is the influence of the emotion of a 

 mother on her offspring. It is still doubtful whether such influence 

 really exists ; but, this theory once established, its bearing on evolu- 

 tion as a prime cause of variation must at once be manifest ; for it 

 gives not only tangibility to the Lamarckian idea of desire influen- 

 cing modification, but also a conception of how infinite mind in 

 nature may act through the finite in directing such modification. 

 In my judgment, this factor acts only when, from whatever cause, 

 and particularly under the spur of necessity, the emotions are ex- 

 ceptionally intensified, or the desire strongly centred in some par- 

 ticular object. 



These psychical factors which we have been considering are 

 substantially Lamarckian ; and in proportion as we consider them, 

 and get to understand the other direct causes of variation, must 



we give importance to the ideas of Lamarck, and, conversely, less 

 importance to the ideas of Darwin. 



There are certain important laws which have influenced modifi- 

 cation, but in no sense can be looked upon as causes of variation. 

 They are laws or principles of evolution by which we may account 

 for the formation of types, acting, just as natural selection does, in 

 differentiating rather than in originating the variation. Acceleration 

 and retardation belong to this class. This law is an attempt to give 

 expression and form to a set of facts to which paleontology un- 

 doubtedly points, and which ontogeny substantiates ; viz., that cer- 

 tain types may attain perfection in time, and then retrogress and 

 finally become extinct, and that existing types which are dying out 

 or degenerating exhibit ontogenically the culmination of force and 

 complexity, followed by decadence, corresponding to the phylo- 

 genic history of the type. This law may, perhaps, be substantially 

 stated in this wise : that certain groups acquire some characters 

 rapidly, while corresponding groups acquire the same characters 

 more slowly, or never acquire them at all ; and this brings us to 

 another important factor of evolution which serves to give force 

 to the law. It is the acceleration by primogeniture which has been 

 elaborated by Hubrecht. He shows, that, in organisms in which 

 the reproductive period covers many years, accelerated develop- 

 ment by primogeniture (i.e., as between the first-born and the 

 last-born of any pair and of their posterity) will in time produce 

 differentiation. The series of the first-born will in the course of 

 time involve many generations at short distances from each other 

 whereas the series of the last-born will, on the contrary, consist of 

 a much smaller number of terms, each separated from its prede- 

 cessor by a more considerable distance. Any tendency to varia- 

 tion from external or internal influences must needs find more 

 numerous occasions to act in the series of the first-born, not only 

 because these have a more composite ancestry, but because they 

 necessarily become the most numerous. 



We are thus led to what have been called ' saltations ' in evolu- 

 tion. Although the history of paleontology has continually added 

 to our knowledge of past forms, and helped to fill up many gaps in 

 the evolutional series, and although during the last quarter of a cen- 

 tury it has particularly vindicated Darwin's prophecy that many 

 links would yet be found, the substantial truth remains, that gaps 

 still occur, and that progress, so far as present knowledge indicates, 

 has been made by occasional saltations. There have been, it would 

 seem, periods of rapid movement, and of comparative repose, or 

 re-adjustment of equilibrium. Cope concludes that "genera and 

 higher categories have appeared in geologic history by more or 

 less abrupt transitions or expression-points, rather than by uniform 

 gradual successions." 



The forces of nature are constant, but the phenomena induced 

 are often paroxysmal. The progressive forces accumulate, while the 

 conservative forces resist until at last resistance gives way with 

 comparative suddenness. There is every reason to believe that the 

 life-movement, in its ascending complexity, has shared this com- 

 mon law. How far the rhythmic tendency in the development of 

 animal life may be explained by the rapid change of climate, by 

 migration and the loss of record, or upon the general law that 

 while there has been progress of the whole there has not neces- 

 sarily been progress of every part, it would take us too far to dis- 

 cuss in this connection. I think we are safe in saying, however, 

 that the facts justify belief that in the evolution of animal life, as in 

 the evolution of every thing else, progress has often been made by 

 waves. 



Having thus considered some of the proximate causes of varia- 

 tion and. some of the more general laws of evolution, we are natu- 

 rally led, in conclusion, to consideration of original or infinite cause. 

 Far be it from me to try your patience with any prolonged specu- 

 lation upon the more profound problems of life and of futurity, 

 which have been dealt with by able men of all times, and with such 

 conflicting and varying results. I shall content myself, in closing, 

 with a few words upon those themes which, as biologists, we can- 

 not ignore, and to which the subjects we have been considering 

 inevitably lead. 



Mind as exhibited in organic evolution, however simple or com- 

 plex may be its manifestations, is in essence one and the same 

 force. There is an undoubted gradation from simple sensitiveness 



