5 o THE POPULAR SCIENCE MONTHLY. 



then the reproductive function is most active. The law may, 

 therefore, be stated thus : The activity of the reproductive func- 

 tion is in proportion to the unfavorableness of the embryonic 

 environment. The following instances are adduced : 



Tape- Worm. The common tape- worm, Tcenia solium, para- 

 sitic 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 swal- 

 lowed 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 de- 

 velopment of the ovum can occur, and it is altogether accidental 

 whether it reaches this situation. The explanation of the enor- 

 mous 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 genera- 

 tions, within a few months. This astonishing fertility is depend- 

 ent 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 



