PROGRESS IN THE STUDY OF VARIATION. 57 



easily accessible forms no collections or experiments have 

 yet been made with a view to answering a question of such 

 peculiar interest to the evolutionist. 



In the following pages I propose to take a few illustra- 

 tions — most of them already well-known to specialists — 

 showing the kind of phenomena which are thus open to 

 observation. The methods that should be applied are 

 essentially the same in each case, consisting in : — 



(1) The collection of large samples taken at random in 



the area believed to be occupied by the pure races. 



(2) Separate seriation of the individuals of each sample 



according to the decree in which the differentiating 

 character is presented. 



(3) Similar collection and seriation of similar samples 



from a series of stations connecting the areas occu- 

 pied by the pure races. 



(4) In suitable cases experimental crosses between the 



pure forms and between each pure form and such 

 intermediates as can be obtained. 



It is unnecessary to say that in planning observations on 

 these lines, special regard must be paid to the possibilities 

 of error due to migrations and other sources of error 

 affecting particlar cases. 



Local races may be distinguished by differentiation in 

 respect of various bodily features, though naturally the most 

 familiar are those which are distinguished by some con- 

 spicuous and easily recognised characteristic, such as 

 peculiar stature or proportions, colour, sculpture, and so 

 forth. But there are several well-known instances where 

 intergradation occurs between races distinguished from 

 each other by what are called "anatomical" characters. A 

 good instance of such a phenomenon is that of a species of 

 Cistucio in N. America, which is represented in New Eng- 

 land by a form with four toes on the hind foot, while 

 Mexican representatives have three. This form was 

 originally considered by Gray as a separate genus, after- 

 wards as a separate species. Subsequent evidence has 

 shown that the fourth toe fades away so gradually that the 



