Parasitism

(6). Deserving consideration by itself is the prolific multiplica tion. Leuckart's estimates have been often quoted, that Taenia solium may produce 42,000,000 eggs in a year, and Ascaris 64,00o, 000. It is possible, no doubt, to discover free-living animals still more prolific, such as the starfish Luidia ciliaris which Mortensen credits with containing 200,000,000 eggs, but there are not many instances of such extraordinary fecundity, even among fishes.

The eggs and larvae of parasites are often subject to severe elimination ; the chances of death are enormous. Therefore in the course of the evolution of parasites, variants in the direction of increased reproductivity would have survival-value.

Some Evolutionist Problems.

(i) It must not be facilely supposed that the adaptive peculiarities of parasites illustrate indi vidual modifications that have hereditarily accumulated until they have become racial characters. Organisms that have begun the parasitic mode of life because of certain constitutional weak nesses may continue to show congruent germinal variations, some of which may have selective value. (2) It must not be too con fidently assumed that all the diagnostic peculiarities of parasites are as such engrained hereditary characters. Many of them may be individual structural reactions to the peculiar conditions of life, which recur with each generation. An organism's characters de

velop as the outcome of environmental, nutritional and functional nurture operating upon hereditary nature. There is much to be said for the view that the pigmy male of Bonellia suffers arrest of development partly because it absorbs the secretion on the pro boscis of the female. More attention should be paid to phenomena like those of "physogastry" in the guests of the white ants, where extraordinary deformations of body come about in probably direct individual reaction to the special conditions of life. But in all such cases a specific capacity for reaction is inherited. Experimental inquiry is also needed to show whether some of the specific char acters of nearly related parasites may not be modificational. Nearly related species in different hosts should be exchanged in early life. (3) It may be noted that many parasites probably illus trate the results of "isolation," which narrows the range of inter-crossing. For not only is the host in some ways like an island, to take the simplest form of "isolation," but the combina tion of circumstances which secure fertilization, dispersal, a second host, and so forth, is often so subtle that it operates as an isolating evolution-factor.