114 SOLLAS Chi the Origin of Freshtvater 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 having proved its superiority In- 

 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 coelenterates 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. (jr. Tubularia. 



