420 DARWINISM chap. 



elaboration that the outer and inner parts of these are 

 necessarily subject to different conditions ; and that the outer 

 actions of air or water lead to the formation of integuments, 

 and sometimes to other definite modifications of the surface, 

 whence arise permanent differences of structure. Although 

 in these cases also it is very difficult to determine how much 

 is due to direct modification by external agencies transmitted 

 and accumulated by inheritance, and how much to spontaneous 

 variations accumulated by natural selection, the probabilities 

 in favour of the former mode of action are here greater, 

 because there is no differentiation of nutritive and reproductive 

 cells in these simple organisms ; and it can be readily seen 

 that any change produced in the latter Avill almost certainly 

 affect the next generation.^ We are thus carried back almost 

 to the origin of life, and can only vaguely speculate on what 

 took place under conditions of which we know so little. 



The American School of Evolutionists. 



The tentative views of Mr. Spencer which we have just dis- 

 cussed, are carried much further, and attempts have been made 

 to work them out in great detail, by many American naturalists, 

 whose best representative is Dr. E. D. Cope of Philadelphia. ^ 

 This school endeavours to explain all the chief modifications 

 of form in the animal kingdom by fundamental laws of growth 

 and the inherited effects of use and effort, returning, in fact, to 

 the teachings of Lamarck as being at least equally important 

 with those of Darwin. 



The following extract will serve to show the high position 

 claimed by this school as original discoverers, and as having 

 made important additions to the theory of evolution : — 



"AVallace and Darwin have propounded as the cause of 

 modification in descent their law of natural selection. This 

 law has been epitomised by Spencer as the ' survival of the 

 fittest.' This neat expression no doubt covers the case, but it 

 leaves the origin of the fittest entirely untouched. Darwin 

 assumes a * tendency to variation ' in nature, and it is plainly 



^ This explanation is derived from Weismanii's Tlieory of the Continuity 

 of the Germ-Plasm as summarised in Nature. 



- Bee a collection of his essays under the title, TJie Origin of the Fittest : 

 Assays on Evolution. D. Appletou and Co. New York. 1887. 



