448 
EMBBYOLOGY. 
Chap. XIII. 
of development, and secondly, from their following 
exactly the same habits of life with tlieir parents; for 
in this case, it would be indispensable for the existence 
of the sjDecies, that the child should be modified at a 
very early age in the same manner with its parents, in 
accordance with their similar habits. Some further 
explanation, however, of the embryo not undergoing 
any metamorphosis is perhaps requisite. If, on the other 
hand, it profited the young to follow habits of life in any 
degree different from those of their parent, and conse¬ 
quently to be constructed in a slightly different manner, 
then, on the principle of inheritance at corresponding 
ages, the active young or larvae might easily be ren¬ 
dered by natural selection different to any conceivable 
extent from their parents. Such differences might, 
also, become correlated Avith successive stages of de¬ 
velopment ; so that the larvs 0 , in the first stage, might 
differ greatly from the larvae in the second stage, as we 
have seen to be the case with cirripedes. The adult 
might become fitted for sites or habits, in which organs 
of locomotion or of the senses, &c., would be useless; 
and in this case the final metamorphosis would be said 
to be retrograde. 
As all the organic beings, extinct and recent, which 
have ever lived on this earth have to be classed together, 
and as all have been connected by the finest gradations, 
the best, or indeed, if our collections were nearly perfect, 
the only possible arrangement, would be genealogical. 
Descent being on my view the hidden bond of con¬ 
nexion which naturalists have been seeking under 
the term of the natural system. On this view we 
can understand how it is that, in the eyes of most 
naturalists, the structure of the embryo is even more 
important for classification than that of the adult. For 
the embryo is the animal in its less modified state; 
