SPONTANEOUS GENERATION AND EVOLUTION : CONCLUSION 379 



be shown in the main that attaining a higher stage in organization 

 also implies a predominance in the struggle for existence, because it 

 opens up new possibilities of life, adaptations to situations not 

 previously utilizable, sources of food, or places of refuge. Thus 

 a number of the lowxr vertebrates ascended from the water to the 

 land, and adapted themselves to life on dry land or in the air, 

 first as clumsily moving salamanders, but later as actively leaping 

 frogs; thus, too, other descendants of the fishes gained a sufficient 

 carrying power of limbs to raise the lightened body from the ground, 

 and so attained to the rapid walk of the lizards, the lightning-like 

 leaps of the arboreal agamas, the brief swooping of the flying-dragons, 

 and ultimately the continuous flight which we find in the flying 

 Saurians and the primitive birds of the Jurassic period, and in the 

 birds and bats of our own day. 



It is obvious that each of these groups, as it originated, conquered 

 a new domain of life, and in many cases this was such a vast one, 

 and contained so many special possibilities, that numerous subordinate 

 adaptations took place, and the group broke up into many species 

 and genera, even into families and orders. All this did not come 

 about because of some definitely directed principle of evolution of 

 a mysterious nature, which impelled them to vary in this direction 

 and in no other, but solely through the rivalry of all the forms of 

 life and living units, with their enormous and ceaseless multiplication, 

 in the struggle for existence. They were, and they are still, forced 

 to adapt themselves to every new^ possibility of life attainable to 

 them ; they are able to do this because of the power of the lowest 

 vital units of the germ to develop numerous variations ; and they are 

 obliged to do it because, of the endless number of descendants from 

 every grade of vital unit, it is only the fittest which survive. 



Thus higher types branched oft' from the lower from time to 

 time, although the parent type did not necessarily disappear : indeed 

 it could not have disappeared as long as the conditions of its life 

 endured; it was only the superfluous members of the parent form 

 that adapted themselves to new conditions, and as, in many cases, 

 these required a higher organization, there arose a semblance of 

 general upward development which simulated a principle of evolution 

 always upwards. But we know that, at many points on this long 

 road, there were stations where individual groups stopped short and 

 dropped back again to lower stages of organization. This kind 

 of retreat was almost invariably caused by a parasitic habit of life, 

 and in many cases this degeneration has gone so far that it is difficult 

 to recognize the relationship of the parasite to the free-living ances- 



