DARTERS FROM LAKE MAXINKUCKEE. 365 



sooner freed from the influence of the lake and, breeding only among 

 themselves, were modified most rapidly by the new environment. In 

 time they became so well differentiated as to be readily distinguishable 

 from the parent form in the lake. 



But during the continuance of the migrations and countermigrations 

 between the lake and the stream, there would be found in the lower 

 part of the stream and in the lake about its mouth the progeny of the 

 individuals from the lake and creek which had interbred. These would 

 possess characters more or less intermediate between the parent species 

 {Etheostoma imvce) and the derived form inhabiting the creek. 80 long 

 as these intermediate forms continued to exist the form found in the 

 creek would be only an incipient species, and as such it would be a 

 subspecies of E. iowce, and would receive a trinomial name. But if, in 

 course of time, invasions of one habitat by individuals from the other 

 should cease, then the intergrading forms would, through interbreeding 

 with the extreme forms, be gradually absorbed by them and finally dis- 

 appear altogether. In the creek would then be found a form differing 

 clearly and constantly from the lake form and without any connecting 

 forms. Under these circumstances the form in the creek, as well as 

 that in the lake, must rank as a distinct species. 



This is the present condition, so far as our investigations have enabled 

 us to determine. There is no difficulty in distinguishing individuals 

 taken in the lake from those found in the creek, and neither form seems 

 to invade the habitat of the other. Large collections were made, not 

 only of the fishes inhabiting the lake, but also of those in the creek. 

 The latter was carefully seined twice from its source to its mouth, and 

 not a single example of E. ioicce or any form showing intergrada- 

 tion was seen. Similarly careful investigations were made in the lake 

 without discovering any individuals of the creek form or any showing 

 intergradation. Whether further collecting will discover connecting 

 forms can not, of course, be stated. The small size of the creek and of 

 the lake, and their close geographic relation, render it almost certain 

 that individuals of the one form would occasionally invade the habitat 

 of the other, and vice versa. While the environment of the creek is 

 markedly different from that of the lake, it is improbable that a change 

 from one to the other would prove disastrous to the individuals con- 

 cerned. Some of such individuals would, it seems, be able to survive, 

 and some would probably interbreed with individuals of the other form 

 whose habitat they had invaded. This was quite likely the condition 

 in the beginning, and the creek form, so long as it remained connected 

 with the parent species by the intergrading forms resulting from such 

 interbreedings, would be a subspecies of the parent species. But, as 

 already stated, no such connecting forms have yet been found, and the 

 form inhabiting the creek is a distinct species. 



There is one other condition worth considering. Let us suppose, 

 after the creek colony had become well established, and for many gen- 

 erations had not intermingled in any way with the parent species in 



