1883.] Growth and Development. 725 
the water, but becomes the lunged Amblystoma if it leaves the 
water. Reproduction takes place in the former stage, though 
it is partly larval. Various other instances of this character 
might be adduced. There are peculiar fishes, the Leptocephali, 
which, by deprivation of normal contact influence, seem to re- 
main embryonic throughout life. They are small, pellucid, rib- 
less, cartilaginous creatures, destitute of generative organs, which 
are found floating far out in the ocean. Gunther considers them 
to be the offspring of various marine fish, which represent an arrest 
of development in an embryonic stage. They have been exposed 
to abnormal conditions, and failed to receive the contact influence 
necessary to call into play the innate energies of development. 
It may be, then, that growth can proceed at any stage of life, but 
that for each new phase of development the animal must be ex- 
posed to new conditions of nature. It has in itself the inherited 
tendencies to successive phases of development until the highest 
is attained, but these remain dormant until set in play by the 
requisite kind of external contact. 
If this be the case, every animal is, to a very.marked degree, 
controlled by the influences of the outer world, growth, activity, 
variation, and the inherent development being all dependant upon 
the instigation of external energy. If we knew the various con- 
ditions to which the ancestral line of any animal had been ex- 
posed, and could reproduce those exact conditions with which to 
surround its offspring, its development might be arrested at vari- 
ous ancestral stages, and its line of evolution made out. The in- 
stances given of retarded development in Amphibia, are cases in 
point. 
An animal species constantly surrounded by one unvarying set 
of conditions will not change. Any tendency to change will be 
restrained by lack of adaptation. Yet natural conditions vary 
not only in kind, but also in degree. Two animals occupying 
the same locality may be exposed to very different natural condi- 
tions. One is played upon by comparatively few of nature’s in- 
fluences, the other by very many, and the complexity of their 
adaptations to nature are in accordance therewith. Thus evolu- 
tion may be of two kinds. One is a change in constitution to 
meet a change in climatic or other conditions. This produces no 
change in rank of development. A second kind of change may 
be either a progression or a retrogression. The animal becomes 
