212 THE EVOLUTION OF SEX 



process and multiplication by special parthenogenetic cells, as 

 is the case in many flukes, we are in the same way bound to 

 suppose that the cells within a sporocyst which give rise to 

 redias are, like ova, charged with this reproductive germ-plasma. 

 It is very interesting to notice that, as far back as 1849, 

 Owen had a distinct prevision, not only of the distinction 

 between body-forming cells and reproductive-cells, of which 

 so much is now made, but of the essential idea of the " germ- 

 plasma." Speaking of the recurrence of a parental form after 

 numerous interpolated generations, he says, " the essential con- 

 dition is the retention of certain of the progeny of the primary 

 impregnated germ-cell, or, in other words, of the germ-mass 

 unchanged in the body of the first individual developed from 

 that germ-mass, with so much of the spermatic force inherited 

 by the retained germ-cells from the parent-cell or germ-vesicle 

 as suffices to set on foot and maintain the same series of 

 formative actions as those which constituted the individua;! 

 containing them." In this somewhat over- weighted sentence, 

 if we read " germ-plasma " instead of " spermatic force," we 

 have a close approximation to the modern conception of 

 Weismann. So again, he says, " an impregnated germ-cell 

 imparts its spermatic power to its cell-oft'spring ; but when 

 these perish, or when the power is exhausted by a long descent, 

 it must be renewed by fresh impregnation. But nature is 

 economical, and so long as sufficient power is retained by the 

 progeny of the primary impregnated vesicle (the essential part 

 of an ovum), individuals are developed from that progeny 

 without the recurrence of the impregnating act." 



§ II. Hints as to the Ratio?iale of Alter?iatio?i. — We shall 

 have to take a fresh view of alternation of generations after the 

 general theory of growth and reproduction has been discussed;, 

 meanwhile, however, the physiological aspect of the facts may 

 be simply indicated. A fixed hydroid contrasted with a 

 swimming-bell or medusoid, a sessile hydra-tuba contrasted 

 with an actively locomotor jelly-fish, illustrate not a peculiar 

 antithesis, but a most general and fundamental rhythm of 

 organic life, — that between nutrition and reproduction. The 

 hydroid has a relatively passive habit and a copious nutrition ;. 

 it is preponderatingly vegetative and asexual. The reverse 

 habit, the physiological rebound, finds expression in the 

 medusoid. In the same way, though the alternation is less 

 strictly between asexual and sexual, the contrast between leafy 



