THE FACTORS OF EVOLUTION 177 



with a fur of a somewhat lighter colour. It seems, however, very 

 doubtful whether this slight difference in coloration would grant 

 a sufficient advantage to enable just these animals to survive. 

 The same observation applies to numerous cases of protective 

 resemblance and mimicry, of which it is equally difficult to 

 understand at the first faint stage why they could be of use to 

 their possessor. But if they were without importance at their 

 first appearance, how was natural selection able to increase 

 them? 



Owing to these objections an attempt has been made, with 

 very little success, to refer the origin of colour-adaptation and 

 protective marking to the direct influence of light and surround- 

 ings. It was thought that, as in colour-photography certain 

 substances present in photographic plates undergo under the 

 influence of coloured light a corresponding change, so we have 

 to regard in the numerous insects this phenomenon as being due 

 to a ' photographic sensitiveness of the skin.' But the colour of 

 the butterfly originates while it is still hidden in the pupa-case. 

 Further, how could the leaf-designs of the Kallima be produced 

 by colour-photography, for, as Plate has pointed out, it would 

 be necessary in that case for, each butterfly to assume for that 

 purpose a certain definite attitude and retain it until the design 

 had been photographed upon its wings? 



' More difficult still is it to explain by the doctrine of selection 

 the appearance of large and comprehensive organs, such as the 

 lungs or wings of the flying saurians and bats. In many cases 

 these are not new formations that is my reason for quoting them 

 but represent a change of functions which the organs have 

 undergone. The lung appears first as a swim-bladder in the fishes, 

 then undertakes in the lung-fishes temporarily, and as it were as 

 an additional office, the duties of a respiratory organ, and finally 

 serves exclusively in that capacity from the amphibians upwards. 

 The origin of the wings of the bat and pterodactylus may perhaps 

 be explained in this manner that at first there appeared in some 

 animals lateral skin-folds which in jumping acted as a 

 parachute. A similar state we find in the flying squirrels 

 (Pteromys alborafus), the taguan (P. petaurista) the assapan (P. 

 volucella) and in a lesser degree in the common squirrel (Sciurus 

 mdgaris). But if such a parachute was already in existence, 



12 



