1895.) Organic Variation. 895 
Certain of the Amphibia present marked instances of the 
influence. of environment as a stimulus to development. A 
tadpole kept forcibly in the water does not become a frog. 
The Axolotl, a gilled salamander, seems to have a power of 
choice in this particular. It continues a water breather while 
it elects to remain in the water, but loses its gills and develops 
into the lung-breathing Amblystoma if it leaves the water for 
a land life. Another interesting instance of this appears in 
the Leptocephali, peculiar larval fishes, small, pellucid and 
cartilaginous, which are found floating far out in the ocean. 
Gunther considers them the offspring of various marine fishes 
which have been swept away from their normal environment 
and their development in consequence arrested. This is, per- 
haps, due to deprivation of the requisite nutriment. 
Many examples of a check to the full development of the 
higher animals, through insufficient nutrition, might be given, 
were it advisable to extend this examination. In the lower 
animals, so far considered, there would seem to be a competi- 
tion between two instincts, one the instinct to devour food and 
move actively, the other the instinct to cease eating and entera 
state of rest. External conditions are, perhaps, only influen- 
tial in giving the precedence to one or the other of these in- 
stincts, though, in most animals, the latter instinct in time 
seems to gain a controlling influence, and development in 
consequence proceeds. 
The instances here given are extreme ones, and are of much’ 
value from their bearing upon the question at issue. Doubt- 
less there are many minor steps of development which need 
no special preparation, and which take place during the ordi- 
nary activities of life. Such steps might be pointed out in the 
invertebrates, while vertebrate development is generally of this 
character, its stages appearing successively without need of 
marked cessation from food or activity. Yet the examples 
adduced are probably exaggerated instances of what always’ 
takes place, a period of nutrition of the organ involved, a tem- 
porary check to nutrition, a diversion of energy to that organ, 
and a more or less rapid developmental change. If this 
change is a considerable one, as in the casting of their shells 
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