Chap. XXIII. CONDITIONS OF LIFE. 271 



the function and occupy the position of leaves, are differently 

 circumstanced with respect to light, &c,, and apparently in con- 

 sequence differ in structure. But, as Mr. Herbert Spencer admits, 

 it is most difficult in all such cases to distinguish between the 

 effects of the definite action of physical conditions and the accumu- 

 lation through natural selection of inherited variations which are 

 serviceable to the organism, and which have arisen independently 

 of the definite action of these conditions. 



Although we are not here concerned with the definite 

 action of the conditions of life on organisms in a state of 

 nature, I may state that much evidence has been gained 

 during the last few years on this subject. In the United 

 States, for instance, it has been clearly proved, more 

 especially by Mr. J. A. Allen, that, with birds, many species 

 differ in tint, size of body and of beak, and in length of tail, 

 in proceeding from the North to the South ; and it appears 

 that these differences must be attributed to the direct action 

 of temperature. 46 With respect to plants I will give a some- 

 what analogous case: Mr. Meehan, 47 has compared twenty- 

 nine kinds of American trees with their nearest European 

 allies, all grown in close proximity and under as nearly as 

 possible the same conditions. In the American species he 

 finds, w r ith the rarest exceptions, that the leaves fall earlier in 

 the season, and assume before their fall a brighter tint ; that 

 they are less deeply toothed or serrated ; that the buds are 

 smaller ; that the trees are more diffuse in growth and have 

 fewer branchlets ; and, lastly, that the seeds are smaller— all 

 in comparison with the corresponding European species. Now 

 considering that these corresponding trees belong to several 

 distinct orders, and that they are adapted to widely different 

 stations, it can hardly be supposed that their differences are 

 of any special service to them in the New and Old worlds ; 

 and if so such differences cannot have been gained through 

 natural selection, and must be attributed to the long con- 

 firmed action of a different climate. 



46 Professor Weismann comes to other authors on the present subject ; 



the same conclusion with respect to for instance, to Kerner's ' Gute und 



certain European butterflies .in his schlechte Arten,' 1866. 

 valuable essay, 'Ueber den Saison- 47 ' Proc. Acad. Nat. Soc. of Phila* 



Dimorphismus,' 1875. I might also delphia," Jan. 28th, 1862. 

 refer to the recent works of several 



