174 THE EVOLUTION THEORY 



cally with the same certainty as in the case of the insect-pupa, or 

 even with any degree of probability, and we must admit that in very 

 many cases, perhaps even in most cases, it is impossible. This is so 

 chiefly because pure palingenesis is hardly likely to occur now ; the 

 ancestral stages were bound to be modified in any case if they were to 

 be compressed into the ever-shortening ontogeny of later descendants, 

 and particularly so if they were to be shunted back into embryo- 

 genesis. In the latter case they would not only be materially 

 shortened, and, as I have already shown, modified by the mutual 

 adaptations of the different developing parts, but time-displacements 

 of embryonic parts and organs would be necessary, as has been very 

 clearly proved by the excellent recent investigations, which we owe in 

 particular to Oppel, Mehnert, and Keibel. A shunting forward or 

 backward of the individual organs takes place — conditioned appar- 

 ently by the decreasing or increasing importance of the organ in the 

 finished state ; for in the course of the phylogeny everything may vary, 

 and not only may a new, somewhat modified, and often more complex 

 stage be added on at the end of the ontogeny, but each one of the 

 preceding stages may vary independently, whenever this is required 

 by a change in its relations to the other stages or organs. Adaptation 

 is effected at every stage and for every part by the process of selection, 

 for all parts of the same rank are ceaselessly struggling with one 

 another, from the lowest vital units, the biophors, up to the highest, 

 the persons. If we reflect that, in the course of the phylogeny of 

 every series of species, a number of organs always become superfluous 

 and begin to disappear in consequence, we can understand what great 

 changes must take place gradually as such a series of phyletic stages 

 is compressed into the ontogeny, for all organs which are no longer 

 used are gradually shifted further and further back in the ontogeny 

 till ultimately they disappear from it altogether. But, while the 

 primary constituents of these ' vestiges ' play their part in ontogeny 

 for a shorter and shorter time, new acquisitions are being more and 

 more highly developed, and thus, in the course of the phylogeny, 

 numerous time-displacements of the parts and organs in ontogeny 

 must result, so that ultimately it is impossible to compare a particular 

 stage in the embryogenesis of a species with a particular ancestral 

 form. Only the stages of individual organs can he thus compared 

 and parallelized. 



But we must not on that account ' empty out the child with the 

 bath,' and conclude that there is no such thing as a ' biogenetic law ' 

 or recapitulation of the phylogeny in the ontogeny. Not only is there 

 such a recapitulation, but — as F. Miiller and Haeckel have already 



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