114 SoLLas—On the Origin of Freshwater Faunas. 
tively few of which survive the chances of destruction which await them in the 
outer world. 
In the higher animals the resources of the parent are less taxed in this 
direction, but more in another—that of providing food for the secluded embryos. 
The parent either contributes yelk to the essential part of the ovum, or in addition 
she lays up with it a store of additional food, such as honey, or a store of 
captured prey, or, as in the case of the Mammalia and some elasmobranch fishes, 
she nourishes it with her own blood. In some cases, when a number of ova are 
left to hatch in the same capsule, one of them haying proved its superiority by 
outgrowing the rest, proceeds to devour them, and thus obtains the requisite 
additional nourishment by feeding on its brothers and sisters. A remarkable 
instance of this occurs in the freshwater genus Hydra—one ovum devouring the 
rest while still in its ovary; so that this Hydrozoon produces only one or two 
young, instead of the countless numbers which are born to its marine relations.* 
Cases of this class are of great interest, partly because they illustrate in a 
strikingly simple manner the supercession of “safety in numbers” by ‘‘ safety in 
secluded development,” and partly since they seem to suggest a return on the part 
of the ova to ‘plasmodial” formation, the stimulating effect of which is so well 
known amongst the Protozoa. 
Thus by providing food for the ovum, the full inheritance of the adult 
organism is secured to the embryo. Herewith a secondary advantage of great 
importance follows to the race. Cells, like complexes of cells, have a life-history of 
their own, bounded on either hand by life and death. These machines for 
converting energy are liable to wear out, to become clogged by residual effete 
products, or perhaps to become converted into some metameric modification under 
the degrading action of constant molecular motion. However this may be, they 
have power to convert only a limited quantity of energy: when they have 
received and expended a definite but unknown amount they cease to work. 
Hence the necessity for the reproductive process. If this assumption be not 
capable of proof, it is at any rate extremely probable. Let us see what it involves. 
A free-swimming embryo which repeats the ancestry spends its time in swimming 
rapidly about by means of its vibratile cilia, in obtaining food and digesting it, 
and while performing these various functions it expends the balance of its resources 
in undergoing structural change. On exchanging the gastrula state for some 
other it has still to work for its own living, and when finally it reaches the adult 
state it has already to a considerable extent worn out its machinery, and expended 
its powers of converting energy. 
In the lower classes of animals, such as the ccelenterates and echinoderms, the 
larval state is not sufficiently prolonged, and the larval changes are not sufficiently 
* The same phenomenon is met with in other Hydrozoa, however, ex. gr. Tubularia. 
