then the reproductive function is most active. The law may, therefore, be stated thus: The activity of the reproductive function is in proportion to the unfavorableness of the embryonic environment. The following instances are adduced:
Tape-Worm.—The common tape-worm, Tænia solium, parasitic in the human intestine, consists in structure of a series, of flat, oblong segments, sometimes eight hundred in number. Each of these segments is sexually perfect, containing both the male and female reproductive organs. The number of ova capable of development in each sexually mature segment is probably not less than five thousand. At this time the segment detaches itself from the others and is discharged from the body. In order that the ova shall develop, it is then necessary that they should gain access to the alimentary canal of the hog. If by chance they are swallowed by this animal, they quickly pass into the larval or cystic stage, burying themselves in the flesh or liver of their host, whence they may be transferred to the alimentary canal of man, where development is completed.
Now, it is obvious that this complex and fortuitous round of life renders the chances of the development of any single ovum exceedingly small. It is not probable, indeed, that one in ten thousand of the ova discharged from the alimentary canal of the host of the mature worm will ever reach the alimentary canal of the host of the larval worm. The embryonic environment is, therefore, in this case, exceedingly unfavorable by reason of its extreme narrowness. There is but one situation in which the development of the ovum can occur, and it is altogether accidental whether it reaches this situation. The explanation of the enormous capacity of the reproductive power in this animal is thus at once apparent. To compensate for the exceedingly narrow chances that the reproductive cell shall survive to complete issue, these cells are generated in excessive numbers.
Aphides.—The aphides are commonly called plant-lice, and are very abundant in summer upon the leaves of most plants. They mature quickly, at least eight or nine generations following one another in a single summer. So prolific are they that it is entirely within bounds to say that a single insect may give rise to several millions of progeny, counting the successive generations, within a few months. This astonishing fertility is dependent upon a peculiar modification of the reproductive process in these insects. During the summer there are strictly no males nor females, but all are sexually alike, and are able to produce ova which develop without fecundation, this exceptional method of reproduction being termed parthenogenesis. Here, then, we have a remarkable variation of the reproductive function resulting in an enormous increase of prolificness. And here again we find an
