BIRDS' EGGS 283 



behaviour and many interests, are less prolific than types 

 with an easy-going life, with abundant food and sluggish 

 habits. But this may simply mean that variations in the 

 direction of economised reproductivity were eminently 

 successful when there were simultaneous variations in the 

 direction of, let us say, keener wits and stronger parental 

 care. But Spencer thought of the correlation as having a 

 direct physiological basis, and this conclusion is less secure. 

 In reference to the familiar fact that moulting hens cease to 

 lay, he said : " While they are expending so much in pro- 

 ducing new clothing, they have nothing to expend for 

 producing eggs." 



Spencer's contrast between the fertility of birds and 

 mammals is very interesting. " Comparing the large with 

 the large and the small with the small, we see that creatures 

 which continually go through the muscular exertion of 

 sustaining themselves in the air and propelling themselves 

 rapidly through it, are less prolific than creatures of equal 

 weights which go through the smaller exertion of moving 

 about over solid surfaces. Predatory birds have fewer 

 young ones than predatory mammals of approximately the 

 same sizes. If we compare rooks with rats, or finches with 

 mice, we find like differences. And these differences are 

 greater than at first appears. For whereas among mammals 

 a mother is able, unaided, to bear and suckle and rear half- 

 way to maturity a brood that probably weighs more in pro- 

 portion than does the brood of a bird ; a bird, or at least a 

 bird that flies much, is unable to do this. Both parents have 

 to help ; and this indicates that the margin for reproduction 

 in each adult individual is smaller." 



It seems to us that the diversity of fecundity cannot be 

 interpreted very satisfactorily in terms of what surplus there 

 may be after satisfying the claims of individuation. The 

 nexus is more indirect. The bat is uniparous not because 

 it spends so much energy in flight, but because variations 

 in reduced fertility were congruent with the flying habit (for 

 a single young one is enough for the flying mother to carry 

 before and after birth), and were favoured because of con- 



