162 DARWINISM TO-DAY. 



in one year than in another, few biologists would draw the con- 

 clusion that the result was due to the selective destruction of those 

 with the longest tails. The more probable explanation would be 

 that those with the shorter tails were in a more advanced stage 

 of their metamorphosis." 



7 Plate, L., "Uber die Bedeutung," etc., pp. 31-32, 1903. 



8 Tayler, J. L., "The Scope of Natural Selection," Nat. Science, 

 Vol. XV, pp. 114-129, 1899. 



9 Morgan, C. Lloyd, "Factors in the Evolution of the Mammalia," 

 Nat. Science, pp. 97-101, 1892. 



10 Plate, L., "Uber die Bedeutung," etc., pp. 159-160, 1903. 



11 Tayler, J. L., "The Scope of Natural Selection," Nat. Science, 

 Vol. XV, pp. 114-129, 1899. I quote as follows: "In the development 



Tayler's ex- * ^ e individual we see a disappearance of structures, 

 planation of de- which appear to become, with advancing development, 

 generation by useless, almost parallel to the gradual disappearance of 

 natural selection, ru( ji me nts, etc., in the history of the species evolution. 

 And a common explanation for both of these series of phenomena 

 can, I believe, be satisfactorily found in the known facts of nutrition. 

 Growth of any tissue would seem to depend on three conditions, 

 a stimulus of the part adequate to promote functional activity, 

 a proper food supply, and efficient removal of products produced 

 by that particular tissue's activity. There is abundant evidence 

 to prove that a tissue tends to degenerate if its own excretory 

 products are not removed; the evil effects produced by fatigue 

 products in muscle and other tissues on the activity of the tissue 

 itself, prove that this factor must be of great importance where- 

 ever it is found to occur. Just as the growth and development of 

 bacteria are interfered with, and finally altogether checked by the 

 accumulation of products of their own activity, so a tissue in the 

 higher organisms has its activity impaired and its power lessened 

 when for some reason diminished elimination of its own metabolic 

 products occurs. Now both in the development of the individual 

 and the race we see an alteration of structure, a gradual transition 

 from the less to the more specialised, and in this gradual transition 

 there must be, as I endeavoured to prove in my answer to the last 

 objection, an alteration in the line of functional activity of the 

 parts, and that, owing to this fact, a tissue that was necessary in 

 the earlier stages became less and less so as specialisation advanced, 

 the whole tendency of the specialising organism being continually 

 and increasingly against the earlier, less specialised, stages. It will 

 thus happen that every structure which is becoming useless, owing to 

 its deficient specialisation, whether in the history of the race or the 

 individual, will have two adverse sets of conditions to contend 



