PEOPLING OF LAND AND SEA 163 



become so rudimentary that they cannot even acquire organs of 

 fixation. The females, on the other hand, possessing reserve 

 substances and a special nutritive aptitude, have resisted these 

 dangers. Yet they have to pass through a very critical period 

 after the metamorphoses which follow in the train of fixation. 

 It is then that their reproductory cells evolve in the direction of 

 the male sex and regain their original sex when the physio- 

 logical equilibrium has been re-established. There are no super- 

 numerary males among the Tunicates, although their evolution 

 has gone much further, since tachygenesis has brought about 

 a regeneration of the free forms ; but their whole history 

 is so much of a pattern with that of the Cirripedes that there 

 can be no doubt as to the identical nature of their case. 



The researches of Maupas l on free-living Nematodes 

 permitted him to report the existence of supernumerary males 

 among certain of these species. I myself 2 have given elsewhere 

 the reasons which lead me to classify the Nematodes not as 

 Worms, according to the usual procedure, but as Arthropods 

 degenerating through inherent inertia into parasitism, like 

 many of the sedentary larvae of Insects. 3 So long as they are 

 parasites, these organisms live in superabundance. Their 

 passage to a free life, which is almost fatal in those 

 groups where the eggs are often hatched in the ground or in 

 the water, leads them back to these precarious food-conditions 

 just as surely as fixation, but by another road. Here, again, 

 there has been a great disturbance in nutrition, and we find 

 the same facts ; males becoming uncommon and inert, then 

 disappearing altogether ; females hermaphroditic, and finally 

 parthenogenetic, if the reproductory cells develop very early 

 through the operation of tachygenesis. 



The organisms which passed from the sea to freshwater 

 streams, lakes, marshes, and damp localities were likewise 

 exposed to distressing uncertainties in the food supply ; and 

 these must have had the same results as in the preceding 

 instances. We have thus the explanation of hermaphroditism 

 in freshwater Annelid Worms, 4 numerous species of Earth- 



1 LV, 463. 



2 LIV, 1345. 



3 Larvae of Coleoptera living in fruit or digging into wood ; larvae of 

 Hymenoptera enveloped and provisioned, or nourished, by their parents ; 

 larvae of Diptera living in organic substances. These are all described in 

 popular language as Worms. 



4 Dero, Nais, Stylaria, Tubifex, Enaxes. 



