64 The Selection Theory 



under water. I might have added many more, for the list of adapta- 

 tions in the whale to aquatic life is by no means exhausted; they 

 are found in the histological structure and in the minutest combina- 

 tions in the nervous system. For it is obvious that a tail-fin must be 

 used in quite a dififerent way from a tail, which serves as a fly-brush 

 in hoofed animals, or as an aid to springing in the kangaroo or as a 

 climbing organ ; it will require quite difterent reflex-mechanisms and 

 nerve -combinations in the motor centres. 



I used this example in order to show how unnecessary it is to 

 assume a special internal evolutionary power for the phylogenesis 

 of species, for this whole order of whales is, so to speak, made up 

 of adaptations', it deviates in many essential respects from the usual 

 mammalian type, and all the deviations are adaptations to aquatic 

 life. But if precisely the most essential features of the organisation 

 thus depend upon adaptation, what is left for a phyletic force to do, 

 since it is these essential features of the structure it would have 

 to determine ? There are few people now who believe in a phyletic 

 evolutionary power, wliich is not made up of the forces known to 

 us — adaptation and heredity — but the conviction that every part of 

 an organism depends upon adaptation has not yet gained a firm 

 footing. Nevertheless, I must continue to regard this conception as 

 the correct one, as I have long done. 



I may be permitted one more example. The feather of a bird 

 is a marvellous structure, and no one will deny that as a whole it 

 depends upon adaptation. But what part of it does not depend upon 

 adaptation? The hollow quill, the shaft with its hard, thin, light 

 cortex, and the spongy substance within it, its square section com- 

 pared with the round section of the quill, the flat barbs, their short, 

 hooked barbules which, in the flight-feathers, hook into one another 

 with just sufficient firmness to resist the pressure of the air at each 

 wing-beat, the lightness and firmness of the whole apparatus, the 

 elasticity of the vane, and so on. And yet all this belongs to an organ 

 Avhich is only passively functional, and therefore can have nothing to do 

 with the Lamarclcian principle. Nor can the feather have arisen 

 through some magical efiect of temperature, moisture, electricity, or 

 specific nutrition, and thus selection is again our only anchor of safety. 



But — it will be objected — the substance of which the feather 

 consists, this peculiar kind of horny substance, did not first arise 

 through selection in the course of the evolution of the birds, for it 

 formed the covering of the scales of their reptilian ancestors. It is 

 quite true that a similar substance covered the scales of the Reptiles, 

 but why should it not have arisen among them through selection? Or 

 in what other way could it have arisen, since scales are also passively 

 useful parts ? It is true that if we are only to call ada})tation what 

 has been acquired by the species we happen to be considering, there 



