VARYING FECUNDITY IN BIRDS.
By , M.A.
In a very interesting article in the December number of 'The Zoologist,' Mr. Basil Davies attempts to explain why some species of birds lay more eggs than others. Personally I feel grateful to him for suggesting this enquiry, and for the reasons he assigns for the remarkable diversity in the number of eggs laid by different species. If, therefore, I criticise to some extent the theory which he propounds, I hope that it will be understood that I do so in no unfriendly spirit.
Mr. Davies compares the reproduction of birds and mammals. He says: "Birds feel it their duty not only to produce a certain number of offspring each year, but also to bring a certain number to maturity." To illustrate this he compares the Cat and the Nightingale. The former breeds at stated periods whether you destroy her offspring or not; but the latter at once prepares to produce a second brood if the first is destroyed. The truth is that the main object of every organism is to reproduce itself. Each species has its own method of bringing this about. The Cat provides for the peopling of the world by future Cats as thoroughly as the Nightingale provides against the extermination of its kind. These facts are familiar to us, but it is not easy to explain them. Under natural conditions the Indian Elephant does not become exterminated, nor the Brown Rat exceed certain limits. On the one hand, with the former the period of gestation is about nineteen months, and rarely is more than one produced at a birth (Roy. Nat. Hist. vol. ii. p. 536; Darwin estimated that though a pair might live to be one hundred years old, their offspring would probably average only six, 'Origin of Species,' 6th edit. p. 51); whereas the Rat bears "four or five times in the year from four to ten blind and naked young, which are in their turn able to breed at an age of about six months, the time of gestation being about twenty days" (Flower and Lydekker's