170 THE INFANCY OF ANIMALS 



due to pigment, and the distribution and deposition of this 

 pigment in the form of patterns of a definite and orderly 

 and constantly recurring type is a much more mysterious 

 matter. Yet if we take a broad survey of the facts we 

 seem to find justification for the view that there is what 

 we may call a natural, inherent tendency for more or less 

 of such pigment to be deposited in the form of stripes. 

 But the primary physiological reason for this longitudinal 

 deposition of the pigment remains quite obscure. 



But however constant the recurrence of this longitudinal 

 pattern may be, in any large number of individuals consti- 

 tuting what we call a " species " — a collection of individuals 

 all possessing the same features — it is never absolutely aUke 

 in any two individuals. It shows a tendency to vary. 

 This " tendency " in each individual again tends to be 

 exaggerated in successive descendants of that individual : 

 just as a stone thrown into a stream creates a series of 

 ever-widening ripples. Commonly this " accident " of 

 pattern confers a benefit on the wearer by causing it to 

 blend or harmonise, more or less perfectly, with its sur- 

 roundings. As a consequence, those individuals which 

 most completely harmonise have the best chance of escaping 

 predatory enemies, and leave the greatest number of 

 descendants which will possess the same fortunate " acci- 

 dent " of coloration. 



But since the pattern is never, as we have remarked, 

 faithfully reproduced, and since each variation or deviation 

 from the type tends to increase in successive generations, 

 such deviations sooner or later assume a critical, vital 

 importance. They may no longer conceal, but advertise 

 the wearer ; and thenceforward all individuals so marked 

 will suffer an ever-increasing toll on their numbers from 

 predatory creatures, and finally that particular pattern 



