APPLICATION TO METAZOA. 127 



like the following hypothesis : The single-celled 

 organism, by means of frequent repetitions, in the 

 course of generations began to attain a larger size, 

 and then to differentiate its external surface into a 

 denser protective layer — the result of stimuli of 

 contact. When such an organism as this would be 

 divided into a number of pieces by the natural 

 process of reproduction, those parts of the proto- 

 plasm which had not undergone a grosser material 

 differentiation would be like the protoplasmic germs 

 of all its ancestors, and capable of responding to 

 the same stimuli, and therefore of developing in the 

 same manner. The only difference between these 

 and the ancestral germs would be the increased 

 complexity of their nervous co-ordinations, as was 

 pointed out in the previous chapter. But, on the 

 other hand, the part of the organism which has 

 been differentiated into the denser outer layer would 

 be in structure so different from the germs of the 

 species that it would be incapable of responding 

 to any of their accustomed stimuli, and therefore 

 incapable of repeating the development ; but if some 

 of the protoplasm remained within the denser pro- 

 tective outside layer, it might continue to live, thus 

 continuing directly the life of the old organism. If 

 all of the organism should be developed into highly 

 differentiated parts or organs, and no part of it be 



