Vol. XXXV.] UG 



good one^ for the physical conditions of Fuerteventura and 

 the Saharan desert are much alike — the light, the climate, 

 the desert vegetation, and possibly the food upon which it 

 exists, being almost the same in both cases. 



Isolation is, perhaps, the strongest factor in the differen- 

 tiation of species, for ouce a bird is cut ofE from the parent 

 stock it can no longer interbreed with it. It immediately 

 " loses its balance," so to speak, and develops certain 

 peculiarities to strengthen it in its struggle for existence. 



Island-forms are often darker than the allied continental 

 race, and this may often be traced to the existing climatic 

 conditions. Many islands are very mountainous in cha- 

 racter, attaining to a great altitude, the mountain-summits 

 being in cloud most of the year ; the result is a moist 

 climate — Madeira is a good example, — and undoubtedly a 

 moist climate produces dark variations in many cases. 



But these conditions do not prevail in Fuerteventura, and 

 we must look for another reason than humidity to account 

 for the dark plumage of the indigenous Bustard. Professor 

 Punnett, who, with Professor Bateson, is the greatest ex- 

 ponent of Mendelism in England, has kindly written to me 

 in answer to my question as to the causes which have 

 brought about this change in the Bustard. I will therefore 

 conclude by reading you his letter, which contains some 

 interesting suggestions. He writes : " I wish that I could 

 1)e of any assistance in the case of the Bustard. This 

 pi'oblem of local races and adaptation is one of the nuts left 

 for us to crack' — if we can. The first thing to be settled is 

 whether the variation is genetic or whether it is merely 

 a direct response to a change in the environment, and only 

 endures as long as the change in the environment endures. 

 In other words, would your Bustard on the island, if 

 removed to the mainland conditions, breed birds like those 

 on the mainland, and vice versa ? 



*'It might in this case be unnecessary to breed them. 

 One has only to recall Beebe's experiments on desert 

 forms, where he was able to get a darkening of the 

 plumage by keeping them in a humid atmosphere during 

 their moult. 



