136 The Classification of Insects 



characteristic basal spots beneath the forewings are 

 sometimes absent, as in P. astrarche, while the black 

 centres of the spots of the under side may disappear, 

 leaving these spots pure white as in the form arta- 

 xerxes. On the other hand the female of P. icarus 

 often exhibits variation away from P. astrarche, and 

 tends to resemble her own mate, the normal brown 

 colour of the upper surface of the wings giving way 

 to a suffusion of the lilac blue. 



The Causes of Variation. — We see, then, that the 

 characters by which two species are distinguished can 

 be traced, at least in their beginnings, to variations 

 arising within the limits of one and the same species. 

 The natural conclusion to be drawn from this is that 

 species are developed out of varieties. Two problems 

 confront us in connection with this question. Firstly, 

 what causes tend to bring about variation .'' Secondly, 

 by what means are the varietal differences fixed and 

 stereotyped so as to give rise to new species .'' 



It is a matter of common knowledge that any animal 

 resembles to a great extent the parents whence it has 

 sprung. This tendency of animals to transmit their 

 likeness to their descendants is familiar to all nowa- 

 days under the name of heredity. Heredity keeps 

 variation within certain limits ; for instance, though 

 the upper wing-surface of a female P. icarus may be 

 largely blue instead of brown, it is never known to 

 be suffused with yellow or red. On the other hand 

 there is, as we have seen, a tendency, often very con- 

 siderable, on the part of the offspring of the same 

 parents to differ from one another. Is this tendency 

 to vary transmitted from parents and ancestors like 

 the tendency to keep to a certain extent constant to 

 type .'' Or is variation due to causes from outside, 

 acting on the animal in the course of its growth ? 

 There seems no doubt that, among insects at least. 



