8 THE POPULAR SCIENCE MONTHLY. 



with connecting ones, would be deemed distinct species, but he found 

 they all passed insensibly into each other. 



Prof. Parsons suggested that more extended observations might 

 connect received species by intermediate forms, no less than so-called 

 varieties ; and Prof*. Gray remarked that the intermediate forms, con- 

 necting by whatsoever numerous gradations the strongly divergent 

 forms with that assumed as a type of a species, so far from disproving 

 existence of varieties, would seem to furnish the best possible proof 

 that these were varieties. Without the intermediate forms they would, 

 it was said, be taken for species; their discovery reduced them to va- 

 rieties, between which (according to the ordinary view), intermediate 

 states were to be expected. 



Recognizing, then, the existence of varieties, and of varieties suffi- 

 ciently pronounced to have led careful naturalists to regard them as 

 distinct species, what shall we say when it is found that these marked 

 forms are correlated with certain physical conditions, many of which 

 have originated within comparatively recent times ? Dr. J. G. 

 Cooper, 1 after a careful study of the California land snails, ascertained 

 that " species, sub-species, and varieties, living in cool, damp situa- 

 tions, become more highly developed (but not always larger) than 

 the others ; the shell assuming a more compact (imperforate) form, 

 and losing those indications of immaturity referred to, viz., sharp, deli- 

 cate sculpture, bristles, and angular periphery. These characteristics, 

 however, remain more or less permanently for indefinite periods, and 

 give that fixedness to the various forms, even when living under the 

 same conditions, which enables us to retain them as sub-species differ- 

 ing from varieties in permanency, and from races in not inhabiting 

 distinct regions." It may be added that Stearns, Bland, and Binney, 

 have likewise observed the same peculiar variations associated with 

 aridity. 



In a broader field, and compassing different classes, Prof. Spencer 

 F. Baird, Mr. J. A. Allen, and Mr. Robert Ridgway, have severally 

 shown that marked and specific changes are seen in birds and mam- 

 mals corresponding to differences in their surroundings. Prof. Baird, 

 in a paper entitled " The Distribution and Migration of North Amer- 

 ican Birds," 2 has shown that birds in high altitudes and those bred at 

 the North are larger than those born South and at low altitudes ; that 

 Western birds of the same species have longer tails than eastern exam- 

 ples, and that the bill increases in size in those birds occurring in 

 Florida as compared with those found north of that State, and that on 

 the Pacific coast the birds are darker in color than those found in the 

 interior. 



Mr. J. A. Allen 3 has made a more special study of this matter, and 



1 "Proceedings of the California Academy of Natural Science," vol. v., p. 128. 



2 American Journal of Science and Arts, vol. xli., January and March, 1866. 



3 "Proceedings of the Boston Society of Natural History," vol. xv., p. 156. 



