IQ 6 THE EVOLUTION OF SEX. 



shown that the reproductive cells, which by hypothesis bear the germ- 

 plasma, often arise far down in the hydroid body, and actually migrate 

 to their final seat in the bearer. Where the alternation is not between 

 sexual and asexual, but between the ordinary sexual 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 rediae 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 distinc- 

 tion 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 condition 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 

 individual containing them." In this somewhat overweighted 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-offspring; but when these perish, or when the power is 

 exhausted by a long descent, it must be renewed by fresh impreg- 

 nation. But Nature is economical, and so long as sufficient power is 

 retained by the progeny <>f the primary impregnated vesicle (the 

 essential part of an ovum), individuals are developed from that progeny 

 without the recurrence of the impregnating act. ' ' 



XI. Hints as to the Rationale of Alternation. — 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 jellyfish, 

 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 nutirtion; 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 spore-bearing fern-plant and incon- 



