LECTUEE IV 



THE COLORATION OF ANIMALS AND ITS RELATION 

 TO THE PROCESSES OF SELECTION 



Biological significance of colours — Protective colours of eggs— Animals of the 

 snow-region — Animals of the desert — Transparent animals — Green animals — Noc- 

 turnal animals — Double colour-adaptation — Protective marking of caterpillars 



Warning markings— Dimorphism of colouring in caterpillars— Shunting back of 

 colouring in ontogeny — ' Sympathetic' colouring in diurnal Lepidoptera — In nocturnal 

 Lepidoptera — Theoretical considerations — The influence of illumination in the pro- 

 duction of protective colouring, Tropidoderus — Harmony of protective colouring in 

 minute details— Notodonta — Objections — Imitation of strange objects, Xylina — Leaf- 

 butterflies, Kcdlima — Hebomoja — Nocturnal Lepidoptera with leaf-markings — Orthoptera 

 resembling leaves — Caterpillars of the Geometridae. 



We have seen what Darwin meant by natural selection, and 

 we understand that this process really implies a transformation of 

 organisms by slow degrees, in the direction of adaptive fitness — 

 a transformation which must ensue as necessarily as when a human 

 selector, prompted by conscious intention, tries to improve an animal 

 in a particular direction, by always selecting the ' fittest ' animals 

 for breeding. In nature, too, there is selection, because in every 

 generation the majority succumb in the struggle for life, while on 

 an average those which survive, attain to reproductive maturity, and 

 transmit their characters to their descendants, are those which are 

 best adapted to the conditions of their life — that is, which possess 

 those variations of most advantage in overcoming the dangers of life. 

 Since individuals are always variable in some degree, since their 

 variations can be inherited by their progeny, and since the con- 

 tinually repeated elimination of the majority of those descendants 

 is a fact, the inference from these premisses must be correct ; there 

 must be a ' natural selection ' in the direction of a gradually increasing 

 fitness and effectiveness of the forms of life. 



We cannot, however, directly observe this process of natural 

 selection ; it goes on too slowly, and our powers of observation are 

 neither comprehensive nor fine enough. How could we set about 

 investigating the millions of individuals which constitute the numerical 

 strength of a species on a given area, to find out whether they possess 

 some variable character in a definite percentage, and whether this 

 percentage increases in the course of decades or centuries? And 



