Various Modifying Agencies. 133 



there are further examples in whieh this fact is verified by an accom- 

 panying visible change in external morphology. 



The fresh water crustacean Branchipus Stagnalis is very much like a 

 salt water species, the Artemia Salina. But the two are placed by 

 naturalists in different genera. The difference is chiefly in the shape of 

 the antennae of the male and the number and form of the posterior seg- 

 ments of the body, there being nine of these segments in the Branchi- 

 pus while the Artemia has but eight. "There is another salt water 

 species of Artemia, the Artemia Milhausenii, which differs considerably 

 from the A. Salina. 



The naturalist Schmankewitsch experimented with these, and by 

 raising several generations of the A. Salina in water, the saltness of 

 which was increased gradually from 4 to 25 Beaume's scale, the 

 descendants of the A. Salina became genuine A. Milhausenii. At the 

 same time by reversing the process beginning with the A. Milhausenii 

 and gradually freshening the water its descendants became A. Salina. 



FIG. 76. Transformation of the 

 tail of the crustacean, Artemia Sa- 

 lina to that of Artemia Milhau- 



XX M^ I \ Tail lobe of A - Salina. 



b, c, d,e. Gradual changes as they 

 breed in water of increasing salt- 

 ness. 



/.Tail lobe of A. Milhausenii. 

 [From Semper after Schmanke 

 witsch.] 



The two species cannot possibly live together in the same kind of 

 water, but the proof is beyond question that they are related by blood 

 and have been differentiated by the action of salt. Schmankewitsch 

 furnishes figures of the gill and tail lobe, showing the steps taken by 

 the animals in this curious transformation. There are six stages in the 

 tail development, each onq inconsiderable, but amounting in the sum to 

 a sufficient difference to justify the specific distinction made. 



But the naturalist went still further, and by gradually freshening the 

 saline water in which the Artemia Salina flourishes, he caused the de- 

 scendants of that, in the course of a few generations, to become true 

 fresh water Branchipi. 



It would appear, then, that the stimulus of the addition of a little salt to 

 the water surrounding the Branchipus, finds expression in subtracting one 

 joint from his tail. From the foregoing results it is easy to understand 

 why marine and fresh water species can be related while neither could 

 be abrupt.lv transferred to the medium <>!' tin* other. The /'itlcmon is a 

 fresh water Crusta<-e;m round often in mountain streams. All of his 

 relatives are marine. In his ^ill cavities he supports a parasite, one 

 Bopyrus, and all the relatives of the parasite are likewise marine, inhab- 



