PRESENT ASPECTS OF EVOLUTION 305 



theory ; and if such an explanation can be made, it 

 would remove one of the strongest objections. But 

 for the present the objection has very great weight. 



Furthermore, as Osborne has insisted, linear varia- 

 tions, or variations proceeding along certain single and 

 weU-marked lines, would seem inexplicable by, if not 

 fatal to, Weismann's theory. And yet Osborne, Cope, 

 and others have shown that the teeth of mammals 

 have developed steadily along well - marked lines. 

 They have apparently not resulted at all by selection 

 from a host of fortuitous variations. 



Says Osborne in his " Cartwright Lectures " * : " It is 

 evident that use and disuse characterize all the centres 

 of evolution ; that changes of structure are slowly fol- 

 lowing on changes of function or habit. In eight inde- 

 pendent regions of evolution in the human body there 

 are upward of twenty developing organs, upward of 

 thirty degenerating organs." Now this parallelism, 

 through a long series of generations, between the evo- 

 lution of organs, their advance or degeneration, and 

 the use or disuse of these same organs, that is, of the 

 habits of the individual, is certainly of great signifi- 

 cance. It must have an explanation ; and the most 

 natural one would seem to be the transmission of the 

 effects of use and disuse. 



On the whole Osborne's verdict would seem just : 

 The Neo-Lamarckian theory fails to explain heredity, 

 Weismann's theory does not explain evolution. But, 

 if the effects of use and disuse are transmitted, corre- 

 lation of variation is to be expected. Muscle, nerve, 

 and ganglion all vary in correlation because they are 

 all used together and in like degree. Evolution and 



* American Naturalist, vols. xxv. and xxvi, 

 20 



