GENESIS. 291 



subsequent re-supply of water. And all along successive 

 generations of parthenogenetic females effect a rapid multi- 

 plication as long as conditions permit. Since life and growth 

 are impeded or arrested not by lack of food only, but by other 

 unfavourable conditions, we may understand how change in 

 one or more of these may set up one or other form of genesis, 

 and how the mixture of them may cause a mixed mode of 

 multiplication which, originally initiated by external causes, 

 becomes by inheritance and selection a trait of the species.* 

 And then in proof that external causes initiate these pecu- 

 liarities, we have the fact that in certain Daphnidece " which 

 live in places where existence and parthenogenesis are pos- 

 sible throughout the year, the sexual period has disappeared : " 

 there are no males. 



Passing now to animals which multiply by homogenesis 

 animals in which the whole product of a fertilized germ ag- 

 gregates round a single centre or axis instead of round many 

 centres or axes we see, as before, that so long as the con- 

 ditions allow rapid increase in the mass of this germ-product, 

 the formation of new individuals by gamogenesis does not 

 take place. Only when growth is declining in relative rate, 

 do perfect sperm-cells and germ-cells begin to appear; and 



* This establishment by survival of the fittest of reproductive processes 

 adapted to variable conditions, is indirectly elucidated by the habits of 

 salmon. As salmon thrive in the sea and fall out of condition in fresh water 

 (having during their sea-life not exercised the art of catching fresh-water 

 prey), the implication is that the species would profit if all individuals ran up 

 the rivers just before spawning time in November. Why then do most of 

 them run up during many preceding months ? Contemplation of the difficul- 

 ties which lie in the way to the spawning grounds, will, I think, suggest an 

 explanation. There are falls to be leaped and shallow rapids to be ascended. 

 These obstacles cannot be surmounted when the river is low. A fish which 

 starts early in the season has more chances of getting up the falls and the 

 rapids than one which starts later ; and, out of condition as it will be, may 

 spawn, though not well. On the other hand, one which starts in October, 

 if floods occur appropriately, may reach the upper waters and then spawn to 

 great advantage ; but in the absence of adequate rains it may fail altogether 

 to reach the spawning grounds. Hence the species profits by an irregularity 

 of habits adapted to meet irregular contingencies. 



