Soitas—On the Origin of Freshwater Faunas. 118 
The Tracheata furnish some interesting exceptions which, like most exceptions 
to a general rule, lend a helping hand to its interpretation. A young cockroach, 
for instance, is born with nearly all the parental characters, yet it is a less special- 
ised form than a butterfly, which starts in life as an humble grub, and long remains 
so. Still, even in the Insecta, we find the highest forms, such as bees and ants, 
postponing a free existence till the larval state is passed. Taken broadly, then, the 
statement that the higher the organism the more advanced the stage at which it 
enters upon a free and independent existence, may be regarded as a sound gene- 
ralization. We may next seek for an explanation of this rule, and we begin by 
the inquiry—What is the use of the more complex organization of a higher form to 
it, and how has it been produced ? Its use, or one use, is to give an advantage in 
the struggle for existence; and it has been evolved by the constant superposition 
of successful inherited varietal modifications. This much being admitted—as it 
is universally admitted by modern naturalists—it is clearly a disadvantage for a 
highly organized animal to produce young which have to start afresh from 
the same level as the inferior competitors, which it has already distanced in the 
race, to repeatedly fight the same battle over again, or to run the gauntlet in its 
ontogenetic development of other competitors in each and every of the less highly 
organized states through which it has passed in its phylogenetic history. As a 
gastrula, it would have to compete with other gastrule, and not with gastrule 
only, but with older and more formidable competitors, more advanced in their 
development than itself. Not only would it be exposed to the dangers of direct 
competition, but also those of the inorganic world—to the violence of currents in 
particular. The very fact that the adult possesses a higher organization is a 
proof of the less efficiency of the lower organization which marks its earlier 
embryological stages. Thus it would be clearly of immense advantage to the 
race for the organism—(1) to abbreviate its larval history ; (2) and to pass through 
that history in a state of seclusion—withdrawn, as far as possible, from the acci- 
dents and competition of the outer world. 
On the higher forms these advantages are always conferred, so that a great part 
of their development takes place in concealment, and many larval stages are passed 
through with surprising rapidity, or even substantially curtailed. 
But the more complete the seclusion of the developing animal, the less the 
possibility of its obtainig food by its own exertions, and hence food must be 
provided for it.* 
In the lower forms of life in which a free larval state is the rule, the resources 
of the parent are greatly taxed in producing a vast number of embryos, compara- 
* Probably the appearance of secondary nourishment in connexion with the ovum was the first 
variation to occur, and secluded development followed as an effect. The advantage which this 
conferred insured more certain survival and continued variation in the same direction. 
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