442 
EMBEYOLOGY. 
Chap. XIII. 
adult: thus Oweu has remarked in regard to cuttle-fish, 
there is no metamorphosis; the cephalopodic character 
is manifested long before the parts of the embryo are 
completed;” and again in spiders, there is nothing 
worthy to be called a metamorphosis.” The larv86 of 
insects, whether adapted to the most diverse and active 
habits, or quite inactive, being fed by their parents or 
placed in the midst of proper nutriment, yet nearly all 
pass through a similar worm-like stage of development; 
but in some few cases, as in that of Aphis, if we look to 
the admirable drawings by Professor Huxley of the 
development of this insect, we see no trace of the ver¬ 
miform stage. 
How, then, can we explain these several facts in 
embryology,—namely the very general, but not uni¬ 
versal difference in structure between the embryo and 
the adult;—of parts in the same indivividual embryo, 
which ultimately become very unlike and serve for 
diverse purposes, being at this early period of growth 
alike ;—of embryos of different species within the same 
class, generally, but not universally, resembling each 
other;—of the structure of the embryo not being closely 
related to its conditions of existence, except when the 
embryo becomes at any period of life active and has to 
provide for itself;—of the embryo apparently having 
sometimes a higher organisation than the mature 
animal, into which it is developed ? I believe that all 
these facts can be explained, as follows, on the view of 
descent with modification. 
It is commonly assumed, perhaps from monstrosities 
often affecting the embryos at a very early period, 
that slight variations necessarily appear at an equally 
early period. But we have little evidence on this head— 
indeed the evidence rather points the other way; for it 
is notorious that breeders of cattle, horses, and various 
