CHAP. IV EVOLUTION THE KEY TO DISTRIBUTION 59 



while in other specimens they are so enlarged as to cover 

 the greater part of the breast and sides of the body, some- 

 times uniting on the middle of the breast into a nearly 

 continuous patch. In one of the small spotted wood- 

 thrushes, Turdus fuscescens, the colours are sometimes very 

 pale, and the markings on the breast reduced to indistinct 

 narrow lines, while in other specimens the general colour 

 is much darker, and the breast markings dark, broad, and 

 triangular. All the variations here mentioned occur be- 

 tween adult males, so that there is no question of differences 

 of age or sex, and the pair last referred to Avere taken at 

 the same place and on the same day.^ 



These interesting facts entirely support the belief in the 

 variability of all animals in all their parts and organs, to an 

 extent amply sufficient for natural selection to work with. 

 We may, indeed, admit that these are extreme cases, and 

 that the majority of species do not vary half or a quarter 

 so much as shown in the examples quoted, and we shall 

 still have ample variation for all purposes of specific 

 modification. Instead of an extreme variation in the 

 dimensions and proportions of the various organs of from 

 10 to 25 per cent, as is here proved to occur, we may assume 

 from 3 to 6 per cent, as generally occurring in the majority 

 of species ; and if we further remember that the above 

 excessive variations were found by comparing a number of 

 specimens of each species, varying from 50 to 150 only, we 

 may be sure that the smaller variations we require must 

 occur in considerable numbers among the thousands or 

 millions of individuals of which all but the very rare species 

 consist. If, therefore, we were to divide the population of 

 any species into three grouj^s of equal extent, with regard 

 to any particular character — as length of wing, or of toes, 

 or thickness or curvature of bill, or strength of markings 

 — we should have one group in which the mean or average 

 character prevailed with little variation, one in which the 

 character was greatly, and one in which it was little, 



^ These facts are taken from a memoir on The Mammals and Winter 

 Birds of Florida, by J. A. Allen ; forming Vol. II., No. 3, of the Bulletin 

 of the Museum of Comparative Zoology at Harvard College, Cambridge, 

 Massachusetts. 



