172 THE POPULAR SCIENCE MONTHLY. 



the common offspring. Now, among the many characteristics thus 

 inherited from both sides in the offsj^ring, there is a sort of struggle 

 for life and a survival of only the fittest and strongest, and thus the 

 cffsjiring improves by the cross. Now, such cross is most completely 

 secured by the separation of the sexes in different individuals — i. e., by 

 unisexuality. 



{p.) Facts which bear on the next Preceding Stex>, viz., the Deriva- 

 tion of Bi sexuality from Asexiiallty. — This is doubtless the most ob- 

 scure step ; yet I believe some light is visible. Here is the greatest 

 gap in the process ; yet this gap may be largely filled. 



Remember, then, that there is a striking correspondence between 

 the embryonic or ontogenic series and the evolution or phylogenic 

 series — that the former is a rapid recapitulation, as it were by memory, 

 of the main points of the latter. The embryo repeats by a kind of 

 organic memory the main point of its descent from primordial proto- 

 plasm. The lesser points, and especially the earliest points, often in- 

 deed drop out of memory, but usually the main points remain. Now, 

 in all the higher animals, ontogeny is a continuous change without 

 break, and completed in one generation. In many lower animals, how- 

 ever, there are apparent pauses, and sudden great changes in this pro- 

 cess of ontogenic development. These are called metamorphoses. In 

 insects, for example, there are two active conditions, the larva and 

 the perfect insect, and a soi*t of second passive egg-stage between — the 

 pupa. Here we have a semblance of, but not really, two generations. 

 Of course, only the perfect insect reproduces. But in many still lower 

 animals we find the metamorphoses occupying two or even more dis- 

 tinct generations. It follows, of course, that in these animals (contrary 

 to what occurs in all higher animals) reproduction takes place both in 

 the larval condition and in the perfect or mature condition. Now, 

 the mode of reproduction in these two conditions is of wholly dif- 

 ferent kinds, the former being non-sexual and the latter sexual. A 

 single example will suffice : The common medusse or jelly-fishes, as 

 you know, are unisexual — i. e., male and female. The fertilized females 

 produce eggs which grow, not into medusae, but into polyp-like ani- 

 mals which are the larval form. These polyps produce buds which 

 open into flower-like bells, then separate and swim away as male and 

 female medusae, which again produce eggs that spring up as polyp- 

 like larvae, etc. Here ontogenesis requires two generations to complete 

 itself. In ontogenesis when both kinds of reproduction occur, the 

 non-sexual (gemmation) precedes the sexual (ovulation). This fact 

 strongly suggests, in fact renders almost certain, that the same is true 

 in phylogenesis, or at least in the phylogenesis of this class. 



But again : Aphids (plant-lice) also rejjroduce in the larval condi- 

 tion, and only reach maturity after many successive generations, some- 

 times as many as nine or ten. In spring these insects are hatched from 

 eggs in a larval wingless condition. From an internal organ analogous 



