196 NAT ORAL SCIENCE. Marcu, 
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 hardly 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 
we examine earlier and earlier stages, and in the earliest stages, 1.¢., 
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, 7.¢., the variation affecting chiefly the adult animal. 
Another kind of variation is illustrated by the three genera of 
enats, Culex, Corethva, and Chivonomus. 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 Culex. The three larve differ not only 
in appearance, but ininternal 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 evolution 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. 
