1907* Notes. 207 



ZOOLOGY. 



The Problems of an Island Fauna. 



It gives me much pleasure to say that I fully agree with Dr. Scharff 

 that the views put forward under the above heading in my address, which 

 he has so kindly criticised in the May number (pp. 178-9, supra), are open 

 to many objections, and that we have not sufficient data, as yet, to prove 

 that a low rate of fertility is apt to prevail on islands. I would, how- 

 ever, take exception to one of Dr. Scharffs arguments— that which he 

 founds on the circumstance that animal life on islands is apt to be abun- 

 dant in individuals — because I cannot think that the fertility of a 

 species has any effect on its abundance or scarcity in a district, except 

 in the extreme case of the fertility falling below the minimum standard 

 necessary to keep the species from dying out. So long as that minimum 

 is ever so slightly exceeded, the species must increase until its numbers 

 are as large as the local conditions will allow to exist ; and this 

 number will be the same whether the fertility is high or low, though it 

 will, from other causes, vary in different countries. From my point of 

 view therefore— and I need scarcely say that it is the point of view of 

 Darwin and of Alfred Russell Wallace, and almost the starting point 

 with the latter of the Natural Selection hypothesis— the fact that a par- 

 ticular species is more numerous on an island than on an adjoining 

 continent is no objection whatever to the suggestion that it may be 

 more fertile on the continent than it is on the island. 



A recent conversation which I had with the Rev. Allan Ellison, on an 

 entirely different subject, supplies me with a curious illustration of the 

 fact I am seeking to urge. Mr. Ellison tells me that in the part of 

 England where he is now resident, the average fertility of the Chaffinch 

 is about 20 per cent, greater than in Ireland. In Ireland the usual 

 number of eggs, according to Mr. Ellison's very extensive experience, 

 is four, and the maximum number five. In Hertfordshire he finds 

 clutches of six quite common, and less than five rare. And yet, Mr. 

 Ellison adds, the Chaffinch is incomparably more abundant in Ireland 

 than it is in England. Here we have a clear case of fertility and abun- 

 dance varying in inver.se ratio, proving, I think, that Dr. Scharffs 

 objection to my argument on this score is not conclusive. It is only fair 

 to add that Mr. Ellison takes an entirely different view from mine as to 

 the reason for this difference in fertility between English and Irish 

 Chaffinches. He thinks that in consequence of small birds being so per- 

 secuted in England a higher rate of fertility has been evolved through 

 Natural Selection to save them from extinction. I doubt whether 

 Natural Selection would work with sufficient rapidity to meet a danger 

 of that kind in that manner. However, all that I am contending for 

 here is that it is possible for a species in a country where its rate of 

 fertility is low— but still sufficient for its requirements — to be more 

 numerous than in another country where its rate of fertility is high. 



C. B. Moffat. 

 Dublin. 



