THE BIOGENETIC LAW 173 



Haeckel was quite well aware of this difficulty, and repeatedly- 

 emphasized it by laying stress on the fact that a ' blurring ' of the 

 phyletic stages of development had arisen through the abridgement of 

 the phylogeny in the ontogeny, and a ' falsification ' of it through the 

 secondary adaptation of individual ontogenetic stages to new con- 

 ditions of life. He therefore distinguished between 'Palingenesis, 

 that is, simple though abridged repetition of the ancestral history, and 

 ' Coenogenesis,' that is, modification of the racial history by later 

 adaptation of a few or many stages to new conditions of life. As an 

 example of coenogenetic modification, I may cite the pupae of butter- 

 flies. Since these can neither feed nor move from one spot, they can 

 at no time have been mature forms, and cannot, therefore, represent 

 independent ancestors of our modern Lepidoptera ; they have 

 originated through the constantly increasing difference between the 

 structure of the caterpillar and that of the moth or butterfly. 

 Originally, that is, among the oldest flying insects, the mature animal 

 could be gradually prepared within the larva as it grew, so that flnally 

 nothing was necessary but a single moult to set free the wings, 

 which had in the meantime been growing underneath the skin, and to 

 allow the perfect insect to emerge, complete in all its parts. This is 

 the case even now with the grasshoppers and crickets. In these 

 forms the larval mode of life differs very little, if at all, from that of 

 the perfect insect, and the main difference between the two is the 

 absence of wings in the larva. But when the perfect insect adapted 

 itself to conditions of life quite different from the larval conditions, as 

 was the case with the nectar-sucking bees and butterflies adapted 

 entirely for flight, while the larvae were still adapted exclusively to an 

 abundant diet of leaves and other parts of plants, and to a very 

 inactive life upon plants, the two stages of development ultimately 

 diverged so widely in structure that the transition from one to the 

 other could no longer be made at a single moulting, and a period of 

 rest had to be interpolated, in order that the transformation of the 

 body could take place. In this way arose the stage of the resting and 

 fasting pupa, a 'coenogenetic' modification of the last larval stage, 

 not a recapitulation of an ancestral form, but a stage which has 

 been interpolated, or better, has ' interpolated itself ' into the ontogeny 

 on account of the widely different adaptations of the early and the 

 final stages. 



This is a perfectly clear idea, and Haeckel's distinction between 

 palingenesis and coenogenesis is undoubtedly justified. 



But it is quite a different matter to be able to decide whether 

 a particular stage or organ has arisen palingenetically or coenogeneti- 



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