No. 579] PROGRESSIVE EVOLUTION 



L59 



ment on the other. If these factors are identical for 

 any two individual organisms, then the behavior of these 

 two individuals must be the same. If the germ-cells of 

 any generation are identical with those of the preceding 

 generation, and if they develop under identical condi- 

 tions, then the soma of the one generation must also be 

 identical with that of the other. 3 Inasmuch as they are 

 parts of the same continuous germ-plasm — leaving out 

 of account the complications introduced by amphimixis 

 — we may assume that the germ-cells of the two genera- 

 tions are indeed identical in nearly every respect; but 

 there will be a slight difference, due to the fact that those 

 of the later generation will have inherited a rather larger 

 supply of initial energy and a slightly greater facility 

 for responding to stimuli of various kinds, for the 

 gradual accumulation of these properties will have gone 

 a stage further. The environment also will .be very 

 nearly identical in the two cases, for we know from ex- 

 periment that if it were not the organism could not de- 

 velop at all. 



Throughout the whole course of its ontogeny the or- 

 ganism must repeat with approximate accuracy the 

 stages passed through by its ancestors, because at every 

 stage there will be an almost identical organism exposed 

 to almost identical stimuli. "VVe may, however, expect 

 an acceleration of development and a slight additional 

 progress at the end of ontogeny as the result of the 

 operation of the law of the accumulation of surplus 

 energy and of the slightly increased facility in respond- 

 ing to stimuli. The additional progress, of course, will 

 probably be so slight that from one generation to the 

 next we should be quite unable to detect it, and doubtless 

 there will be frequent backslidings due to various causes. 



We can thus formulate a perfectly reasonable explana- 

 tion of how it is that the egg first undergoes segmenta- 

 tion and then gives rise to a blastula resembling a hoi- 



