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THE AMERICAN NATURALIST [Vol. XLVIII 



A priori it seems improbable that less variety will be found 

 among the various internal systems of organs than in the in- 

 tegumentary or exoskeletal parts. But an argument may be 

 put forward that the external parts in immediate contact with 

 the environmental forces would be the first to register change in 

 tht 1 modification of a species. The internal parts as stanchions 

 and bulwarks remain firm to give characters to orders and fami- 

 lies, while change makes assault without and gives characters 

 for species. For example, among the Asteroids it is said that 

 the internal organization is so uniform that the only method of 

 classification is to take the different ways in which the demands 

 of the external environment have been met. 



But generally speaking a species depends for its survival not 

 simply upon the external front it presents to its environment. 

 An animal's form cannot arbitrarily be divided into external 

 and internal parts. It is an integral whole, and variation and 

 selection may occur anywhere, while the correlation of variation 

 is a text-book commonplace. As opposed to correlative variation 

 there is the law of the independent variation of parts. Not only 

 may variation occurring in one part cause a variation to take 

 place in another, but variation may take place independently in 

 some areas and be limited in another, so that in deciding upon 

 the comparative value of the internal and external parts in any 

 group consideration must be given to both these laws. In the 

 Asteroids, we assume that anatomists have t ik n the itter in 

 hand and found that the external parts vary as a rule independ- 

 ently of the internal which remain constant, But in how few 

 groups has such a precaution been taken ! Is it not rather the 

 general rule simply to assume that the internal parts lack varia- 

 tion and are of no value systematically, as. for instance, in the 

 Lepidoptera. where the Lepidopterists expect that a classification 

 based upon the wing-markings or upon wing-neuration can ex- 

 press the true relationship of the various units? 



Even in those groups where systematists have dissected and 

 found the internal parts valueless it still remains necessary, in 

 view of the law of independent and unexpected variation of 



It is impossible to deny that the external parts are often of 



and develop color patterns (although color is usually an unsafe 

 guide if taken alone), and the external parts of such forms as 



