194 BIOLOGICAL LECTURES. 



of the capacity for regeneration followed the adaptation of a 

 limb to a new function." 



Weismann had, in his book on the Germ Plasm, committed 

 himself to the statement that the germ plasm for regeneration 

 is different from that which brings about the development of 

 the egg. He believed that it was necessary to make this 

 hypothesis to account for the appearance of the so-called "an- 

 cestral organs " that are sometimes (?) regenerated. Combin- 

 ing this statement with what has been quoted above, it will be 

 clear that as a new species evolves from an older one its regen- 

 erative germ plasm must also change if the animal regenerate 

 a part like the one lost. Hence each species must acquire the 

 power of regenerating its own particular kind of structures. 

 This opinion I did ascribe to Weismann, and still suppose he 

 holds to it. I did not imply that regeneration " must be 

 inscribed, so to speak, on a tabula rasa." The capacity to 

 regenerate the parts of the old species would be present in the 

 new one, but the new species must acquire through natural 

 selection of favorable variations the power to regenerate the 

 new structures that have arisen through egg variations. I have 

 quoted at length this argument of Weismann to show where we 

 are landed by the results of his speculation. He argues for a 

 double process of natural selection for each species that can 

 regenerate, and is led into this position by the assumption that 

 seems quite necessary on the preformation hypothesis that 

 regenerative germs exist in the egg independent of the germs 

 of embryonic development. 



I find in this whole argument only an attempt to shift the 

 difficulties of the problem back to the unknown ancestors of 

 present forms, just as the difficulties of other parts of the prob- 

 lem are also shifted back upon the unknown germs that exist 

 preformed in the egg or in the parts that can regenerate. 

 Weismann does not, perhaps, realize the difference between 

 himself and those whom he somewhat scornfully calls " the 

 younger investigators." The problems that they are trying to 

 solve are those that Weismann also tries to answer, but " the 

 younger investigators " base their interpretations on the assump- 

 tion that when a change takes place a sufficient cause for the 



