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NATURAL SCIENCE. March. 



these several breeds were placed in a row, though most of them could 

 just be distinguished, the proportional differences in the above 

 specified points were incomparably less than in the full-grown birds. 

 Some characteristic points of difference — for instance, that of the 

 width of the mouth— could hardl)^ be detected in the young. But 

 there was one remarkable exception to this rule, for the young of the 

 short-faced tumbler differed from the young of the wild rock pigeon 

 and of the other breeds in almost exactly the same proportions as in 

 the adult state." (" Origin of Species," 6th edition, p. 392.) 



As, it seems to me, would be expected, the differences between 

 two more-distantly related animals at a stage corresponding to the 

 time of hatching in pigeons is very much greater. A bird is obviously 

 a bird at the time of hatching, and often the species and even the 

 breed (as in the case just quoted from Darwin) are readily recognised 

 at that time. A lizard or other reptile at the corresponding stage is 

 easily recognised as a reptile. As shown by Von Baer the distinction 

 between the bird and the reptile becomes more and more difficult as 

 w^e examine earlier and earlier stages, and in the earliest stages, i.e., 

 in the ovum, it is often quite impossible to say even to which great 

 phylum of the animal kingdom the ovum belongs. 



The variation which produced birds from reptilian ancestors 

 may be taken as a type of the kind of variation which is most 

 familiar to us, i.e., the variation affecting chiefly the adult animal. 



Another kind of variation is illustrated by the three genera of 

 gnats, Cnlex, Corethra, and Chironomus. Whatever may have been 

 the form and structure of the most recent common ancestor of these 

 three, the variation which has produced these from that ancestor 

 appears to have affected the larval stages more markedly than the 

 adult stages. These three are quite unlike in appearance in the larval 

 stage. The mode of formation of the imago within the larva in 

 Chironomus is quite unlike that in Ciilex. The three larvae differ not only 

 in appearance, but in internal structure. In the adult stage, though 

 easily distinguishable, they are very much more alike in form, and 

 even in internal structure, than in the larval stages. 



We may not know the exact course of e\olution of these three, 

 but we may at least say that the variation in the average structure 

 which has occurred in their evolution has led to a greater difference 

 in larval structure than in adult structure. 



Two kinds of variation must, therefore, be recognised — that 

 affecting chiefly the adult structure, and that affecting chiefly the 

 structure of the individual in early stages of development. 



My object now is to show that in neither case can a record of the 

 variation at any one stage of evolution be preserved in the 

 ontogeny, much less can the ontogeny come to be a series of stages 

 representing, in proper chronological order, some of the stages of 

 adult structure which have been passed through in the course of 

 evolution. 



