140 PROBLEMS OF EVOLUTION 



principle constantly at work : no species has complete freedom 

 to wander ; 'it is restrained by heredity. Though it may deviate, 

 it may not go beyond certain limits. Thus primitive reptiles 

 had before them at least two widely different possibilities, avian 

 and specialised reptilian lines of evolution, whereas a species of 

 bird, if evolution proceeds further, can only modify the avian 

 type or the type of its particular species or family. It may, in 

 fact, become a new kind of bird, even a bird in which some of 

 the avian characteristics have lapsed, but complete retrogression 

 replacing it among the reptiles, is almost, if not quite, an 

 impossibility. The amphibian type is even further out of reach. 

 As we ascend it will be seen that the lines along which species 

 may develop, though certainly not less in number than lower 

 down in the scale, are less divergent one from another. A 

 primitive bird, as the birds of to-day show us, was capable of 

 evolving an enormous number of specialised forms, but the 

 distance between any two of these is as nothing compared with 

 the distance between the descendants of primitive protozoa. 

 The more elaborate and complex the organism, the narrower 

 the limits within which further evolution must proceed : the 

 narrower the limits within which play is given to mere 

 chance. 1 



Though I have tried to guard myself against it, I may have 

 seemed to be speaking of known forms as the only possible ones. 

 I am far from thinking that. The known species, living and 

 fossil, may be far less in number than those of which we have 

 neither record nor living representatives. I only maintain that 

 every species, at whatever stage of evolution, has a limited 



1 Eimer Organic Evolution, p. 52 writes: "I assume with him (Nageli) that 

 the condition for a progress towards the more complex and towards division of 

 labour exists in the fact that a higher stage once reached can afford a foundation 

 for one still higher, since the former, the existing stage, will necessarily be the 

 starting point for further modification." When reading his book some few years 

 ago, I passed over this passage, failing to realise its importance. I have now 

 arrived at what would seem to be the same position independently, or, it may be, 

 have recurred unconsciously to what I read there. But Nageli, to whom Eimer 

 refers, regards Natural Selection as at best only an auxiliary principle. And Eimer 

 is a strong Lamarckian. My views, therefore, are very different from theirs, though 

 they meet at one point. See Weismann's Essays, vol. i. pp. 262, 306. 



