3o8 THE EVOLUTION OF SEX. 



progeny of a parent stock some were fertile inter se, but infer- 

 tile with the consistent members of the parent stock; these 

 will be isolated by a physiological barrier, just as they might be 

 insulated by a geographical one, and left free to develop along 

 divergent paths of their own. Here again there is recognition 

 of the reproductive factor in evolution ; but how far, and in 

 what cases species have so originated, is obviously a ques- 

 tion which would involve discussion of each individual 

 instance. 



(d.) Worthy of reiteration is the suggestion of Robert 

 Chambers, crudely illustrated as it may have been, that 

 environmental influences acted with special power upon the 

 generative system, and that the prolongation of gestation was a 

 maternal sacrifice which brought its own reward in the higher 

 evolution of the offspring. Miss Buckley, along a similar line 

 of thought, has well pointed out how the increase of parental 

 care was a factor in, as well as a result of the general ascent ; 

 how the success of birds and mammals especially must in part 

 be interpreted in reference to the noteworthy deepening of 

 parental affection, and strengthening of the organic and emo- 

 tional links between mother and offspring. In emphasising the 

 progressive value of prolonged infancy, especially in the evolu- 

 tion of the emotions, Fiske has also recognised the importance 

 of the reproductive factor. 



§ 3. Further Cofistructioti. — The general tendency of all 

 theories of evolution has been to start with the individual 

 organism as the unit, and to consider the self-maintaining and 

 nutritive activities as primary, the reproductive and species-re- 

 garding as only secondary. Butalong many lines of research, such 

 as those indicated in the preceding paragraph, the importance 

 of the reproductive factor has been recognised, and the centre 

 of gravity of the evolutionary inquiry has already been to some 

 degree shifted. Recent investigations on heredity, for instance, 

 forbid that attention should any longer be concentrated on the 

 individual type, or reproduction regarded as a mere repetition 

 process ; the living continuity of the species is seen to be of 

 more importance than the individualities of the separate Hnks. 

 Physiologists and evolutionists are coming to see the most 

 complex individual hves, in Foster's phrase, as " but the bye- 

 play of ovum-bearing organisms." The species is a continuous 

 undying chain of unicellular reproductive units, which indeed 

 build out of and around themselves transient multicellular 



