310 THE EVOLUTION THEORY 



with sufficient precision to be able to say definitely whether the 

 differences between two nearly- allied species are or are not adapta- 

 tions to different conditions. The same is true with reo-ard to the 

 other side of the ^^robleni — the conditions of life. These are often 

 to all appearances identical in two allied species, but even where 

 they are visibly different it is often difficult to assert that the 

 differences between the two species can be interpreted with cer- 

 tainty as adaptations to the specific conditions of life. At an 

 earlier stage we discussed the protective coloration of butterflies, and 

 we saw that the forest butterflies of the Tropics frequently mimicked 

 a dry leaf on their under surfaces. In the various regions of the 

 extensive forest districts of the Orinoco and the Amazon in South 

 America there are fifty species of the genus Ancea alone, and in the 

 resting pose all these bear a most deceptive resemblance to a leaf, yet 

 each of them differs from the rest in the mingling of its colours, its 

 brilliance, and usually in markings Avhen these are present. If we 

 wished to be able to decide whether these specific differences were of 

 an adaptive nature or not, we should first of all require to know in 

 Avhat kind of forest two neighbouring species lived, and in what 

 places, among what sort of leaves, they were in the lialjit of settling. 

 Even then we should at best only know whether the species A was 

 better protected, as far as our own eyes were concerned, among tlie 

 leaves of the forest A^ than the species B, and conversely ; but we 

 could not tell whetlier they required this protection, or whether the 

 species A, if transferred to the forest B', would be more frequently 

 discovered and destro^^ed by its enemies than in its own forest-home, 

 and that alone could prove the difference to be biologically important, 

 that is, to have selection-value. The difficulty, indeed the impossi- 

 bility, of arriving at such decisions can perhaps be better illustrated by 

 an example from our indigenous fauna. No one doubts that the upper 

 surface of the anterior wing in the so-called banner-moth (Catotala) 

 possesses a very effective protective colouring ; by day the moths rest 

 Avith wings spread out flat upon tree trunks, wooden fences, walls, &c., 

 and they are so excellently suited to their environment that they are 

 usually overlooked both by man and animals. But each of the twelve 

 German species of Catocala has a special protective colouring ; in Cato- 

 ccda fraxini it is a light gre}^ in Catocala nwpta, a dark ash-grey, in 

 Catocala elocata rather a yellowish-brown gre}^ in Catoccda spoiua an 

 olive brown, in Catocala prom I tisa a mingling of whitish-gre}^ and olive 

 brown, and so on. All these colourings are protective ; but could any 

 even of our most experienced and sharp-sighted entomologists prove 

 that each of these different shades of colour depends upon adaptation 



