EVOLUTION 1033 



the adult females, and then in the young forms. Kammerer reports 

 that on some islets all the adult males had varied in a certain direc- 

 tion, but only half of the adult females, and none of the 3^oung ones. 

 It is thus easy to understand that an inspection of the lizards with- 

 out analysis of sex and age would give an erroneous impression of 

 what was going on. And this masking of orderliness in variation 

 may be increased by a fact which Eimer noticed in 1881, that the 

 change in the dorsal ground-colour of a varying individual works 

 from the tail-region towards the head (Elmer's "Law of postero- 

 anterior evolution"). Another complication is the occurrence of 

 conservative individuals or even strains which do not share or are 

 at least laggards in the general movement [Riickstdnde) , and of 

 reversions (Riickschldge) which hark back to an antecedent phase, 

 especially to that seen in the young forms. It must have required 

 very careful study to distinguish between the "laggards" and the 

 "throw-backs". As the variation proceeds towards completion 

 (e.g. total melanism) the individual reversions and transilient 

 mutations become rarer and disappear; and Kammerer believes that 

 this holds for other characters besides colouration. 



What is the precise influence of the insulation ? In the first place, 

 there are no peculiar "island forms". In the second place, while 

 the same variants may occur on the mainland and on islands, 

 the range and frequency of variation is much greater among the 

 islands. In the third place, though at first it may seem paradoxical, 

 the island lizards frequently show a persistence of old-fashioned 

 (palingenetic) features along with new ones. In the fourth place, it 

 seems practically impossible to correlate the variations established 

 in different islands with peculiar conditions which would afford 

 opportunity for the action of natural selection. 



To take the last point first, these lizards have almost no enemies, 

 except one another. In crowded places, which are common, the 

 young forms are often devoured; but variation is least seen among 

 the young. Dark pigmentation, may protect against too intense 

 illumination and light colouration sometimes makes the lizard 

 inconspicuous against its background, but the variations are too 

 manifold to admit of more than occasional interpretation in terms 

 of natural selection. 



Kammerer's general position is: (i) that the conditions of life on 

 the Dalmatian islands are very diverse: as regards size, soil, surface- 

 relief, colour of rocks, humidity, temperature, illumination, and 

 vegetation, (2) that the environmental diversity affords numerous 

 liberating stimuli to variability, and that environmentally induced 

 changes may be hereditarily entailed and increased ; and (3) that the 

 insulation serves automatically to accelerate the process of fixation 

 by narrowing the range of inter-crossing. But besides being a 

 cradle for what is novel, an island may occasionally serve as a 



