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THE AMEBIC AN NATURALIST [Vol. XLIX 



even if there were no such thing as natural selection, that 

 the slow successive variations in this case are not chance 

 variations, but due to a fundamental property of living 

 protoplasm and necessarily cumulative. 



Moreover, the accumulation of surplus energy in the 

 form of food-yolk is only one of many habits which the 

 protoplasm of the germ-cells may acquire in a cumu- 

 lative manner. It may learn by practise to respond with 

 increased promptitude and precision to other stimuli 

 besides that of the presence of nutrient material in its 

 environment. It may learn to secrete a protective mem- 

 brane, to respond in a particular manner to the presence 

 of a germ-cell of the opposite sex, and to divide in a 

 particular manner after fertilization has taken place. 



Having thus endeavored to account for the fact that 

 progressive evolution actually occurs by attributing it 

 primarily to the power possessed by living protoplasm 

 of learning by experience and thus establishing habits 

 by which it is able to respond more quickly to environ- 

 mental stimuli, we have next to inquire what it is that 

 determines the definite lines along which progress mani- 

 fests itself. 



Let us select one of these lines and investigate it as 

 fully as the time at our disposal will permit, with the 

 view of seeing whether it is possible to formulate a 

 reasonable hypothesis as to how evolution may have 

 taken place. Let us take the line which we believe has 

 led up to the evolution of air-breathing vertebrates. 

 The only direct evidence at our disposal in such a case is, 

 of course, the evidence of paleontology, but I am going 

 to ask you to allow me to set this evidence, which, as you 

 know, is of an extremely fragmentary character, aside, 

 and base my remarks upon the ontogenetic evidence, 

 which, although indirect, will, I think, be found sufficient 

 for our purpose. One reason for concentrating our at- 

 tention upon this aspect of the problem is that I wish 

 to show that the recapitulation of phylogenetic history 

 in individual development is a logical necessity if evolu- 

 tion has really taken place. 



