OKOTTHOLOGIST 



[Vol. 15-No. 1 



The same generalization with a few more 

 variations is equally applicable to specimens 

 from east to west, eggs becoming larger to the 

 westward, with the exception of that part 

 occupied by the arid region of Arizona and the 

 western part of New Mexico and Colorado, and 

 among highly developed birds — the narrow 

 belt of land on the Pacific coast and lying be- 

 tween it and the nearly parallel Coast Range. 



Those familiar with the topographical map of 

 the United States will remember that there is a 

 gradual increase of altitude in an advance 

 from the Appalachian Mountains which form 

 the western boundary of the Atlantic C^oast 

 district to the western base of the Rocky 

 Mountains, while the territory situated be- 

 tween the latter and the Coast Range, forms a 

 vast plateau, and is the highest table-land in 

 North America, and the narrow belt west of the 

 Coast Range is comparatively on the sea level. 



Thus we find that with the exception of that 

 part of the Central Plateau in the immediate 

 vicinity of Arizona, the increase in altitude cor- 

 responds to the increase in size of eggs as 

 shown by the data already given. Having seen 

 the effect of latitude and altitude upon the size 

 of eggs, let us see what plausible explanation 

 can be offered as the cause of such effect. 



We know that an increase in latitude and 

 altitude cause an inverse change in tempera- 

 ture. We also have found that eggs increase 

 in size in proportion to latitude or its equiva- 

 lent, and we know that the same phenomenon 

 is noticeable in the birds themselves, and the 

 coincidence of these facts would seem to be a 

 signification of some relation between them. 



As it is impossible that the size of a bird or 

 egg lias any effect from climate or temperature 

 we infer that if a co-relation exists, size must 

 be modificable by latitude or the effect of lati- 

 tude upon climate. . 



According to the theory of evolution the 

 laws of natural selection and individual varia- 

 ti(m wt)uld be important factors in the solution 

 of the question. 



It is believed that at the advent of the last 

 Glacial period, birds in common with other 

 animals were obliged to retreat to the south 

 and the migrants, added to the number already 

 inhabiting the regions free from ice, caused an 

 overstocking of that territory, to such extent 

 that a competition fierce enough to tax every 

 energy to its uttermost to find subsistence was 

 the result. 



Under such circumstances the receding (»f 

 the ice was rapidly followed up by a few intent 

 upon reaching a place free fiom competition. 



and these pioneers, being joined by others of 

 their kind, again pushed on in the advance. 

 As they returned to the north, the difference 

 in climate, having less effect upon the more 

 robust individuals, the point in their favor 

 placed them above their associates in the 

 struggle for existence, and the beneficial 

 change was kept up and extended by natural 

 selection, so that in time the superiority of the 

 individual became a trait common to all. 



If such variation will occur among birds 

 when sufficient cause is given, it is equally 

 probable that the eggs of birds may be infiu- 

 enced in the same way. 



We will suppose that when the videttes of 

 the force advancing northward reached a 

 higher latitude than the mass of its kind, it 

 chanced to produce eggs a little larger than 

 the average. As the axiom "Large bodies 

 cool more slowly than those of smaller size"' 

 can not be disproved, it will at once be seen that 

 such eggs would have been better able to with- 

 stand the greater vicissitudes of climate conse- 

 quent to the increase of latitude, or in other 

 words the case of individual variation resulted 

 favorably to the egg, .and the chick when 

 hatched might have been more developed in 

 its several parts than others, and finally become 

 a larger bird than the average. A larger and 

 better organized bird could have withstood 

 more hardships, but it is very probable that it 

 would have required more food than a smaller 

 one, and in the regions quitted, this would have , 

 been a serious drawback, but in its new envi- 

 ronment, competition would have been less 

 severe and so allowed a greater food supply. 

 Thus the variation in the egg would have been 

 a permanent benefit, and only permanent bene- 

 fits are fostered by natural selection. 



It is to l>e remembered that envinmment 

 determines the worth of variations, and had a 

 change like the above occurred in the crowded 

 district it svoidd have been speedily blotted 

 out, as such variation in an egg would not 

 have been of any particular advantage in a 

 warm climate, and the increased amount of 

 food required by the bird would have been ex- 

 tremely difficult to obtain. 



The measurements show an increase of size 

 in uniscm to that of latitude, as before stated, 

 and my conclusion is that the two facts must 

 be related as I have described, the api)arent 

 contradictory evidence in the smallness of 

 Xew Mexican and Arizonian eggs, being due 

 to the extreme heat of summer in that locality 

 which counterbalances, to a measure, the ef- 

 fects of altitude. Ilarrn Gordon White. 



