196 JOURNAL OF SCIENCE. 
evidently so nearly related. -All decapod larve are freely- 
swimming animals, gaining their own living by an active search 
for food ; they are therefore subjected to a struggle for existence 
precisely similar to that of adult animals. The principle of 
natural selection will be as potent to select and modify them as 
it is in selecting and modifying adults. If therefore we assume 
the protozoea as an original form, we must expect to find it in 
many cases highly modified, and must expect in most larvee to 
find, not a protozoea, but a greatly different form, and one better 
adapted for the struggle for existence. Nor must we be surprised 
if the embryologist comes to the conclusion that the modified 
larval stages do not represent stages of ancestral history. 
That the protozoea larva is not well adapted for a struggle 
with numerous enemies is evident to any one who observes how 
easily it is captured. Assuming that this is the early larval 
form, we should not expect from what we know of the workings 
of nature that such an evidently weak form would be preserved, 
except in isolated cases. To adapt such a larva to a more 
effective struggle, there are three methods: the larvee may be 
largely increased in numbers, which would, of course, increase the 
chance of the species for survival ; or they may develop powers 
of flight which will enable them to escape their enemies ; or the 
larvee may develop some sort of defensive armor, which will 
enable them passively to resist all ordinary attacks. Abundant 
examples of each of these methods may be found in almost any 
group of the animal kingdom, but probably no better instances 
than the larvae in question ; and this is all the more interesting 
since it shows that some of the principles affecting adults also in 
a similar way have their influence on larve. With these points 
in mind, it is possible to explain all of the important differences 
between the protozoea and the two zoea types. 
What explanation can we find for the shortened body? Two 
explanations for this can be found, both of which probably had 
their influence. The possession of such a long, weak, almost 
functionless hind-body as is found in the protozoea is certainly 
calculated to render its possessor a more easy prey to enemies 
than it would be were the body more compact. The shortening 
may therefore be simply a protective measure. Or a second 
principle has probably had even more influence. There is good 
reason for believing that the amount of energy of a developing 
animal is limited, and, if expended in one direction, cannot be 
employed in a second. If, for example, a child over-develops its 
brain, its body is sure to suffer. Now this principle has had a 
similar effect in our larve. In the protozoea the energy of de- 
velopment is evenly distributed to all parts of the body. The 
result is, that we find here a larva with almost all of the body 
present, but in a low state of development ; the larva is con- 
sequently comparatively weak. If, however, the development of 
a part of the body should be postponed, the parts which did de- 
velop could reach a greater state of perfection, since the whole 
energy of development could be directly turned toward their 
