94 ORTMANN — AFFINITIES OF CAMBARUS. [April i 3 , 



first and second group are known to me, where the question is ad- 

 missible, whether convergency plays a part. There are rarely two 

 species known, where the shape of the male organs is absolutely 

 identical (except in the third group of Faxon, where they are re- 

 markably uniform), and since they have developed from the begin- 

 ning in three or four different main lines, it is easily understood 

 why they furnish the best specific characters as well as the best 

 criteria for judging the affinities. Thus the danger of being misled 

 by convergency of structure, which is the chief impediment of 

 properly recognizing natural affinities in any group of animals, is 

 here reduced to a minimum. We shall see below that by the ac- 

 tual use of this principle we arrive at conclusions that render the 

 investigation of the development of the genus Cambarus a com- 

 paratively easy task, furnishing a clue to the explanation of the 

 geographical distribution ; further, the study of the male organs 

 gives us a standard by which to judge the other characters that are 

 of systematic value, and as we shall presently see, there is hardly 

 another structure that has the same value for revealing the affinities 

 within the genus, that is to say, the same characters generally de- 

 velop independently in different groups, being clearly subject to 

 parallelism, presumably under the influence of similar external con- 

 ditions. In a few cases the latter is very evident. 



Copulatory Hooks of the Male. — Faxon lays much stress upon the 

 number of hooks present in the male on the ischiopodite of the sec- 

 ond, third, or fourth pereiopods, which are used to take hold of the 

 female in copulation. The third pereiopods always possess these 

 hooks, and in many cases only this pair is present. But sometimes 

 there is an additional pair on the second, or on the fourth pereio- 

 pods. The number of pairs of hooks is very constant in the single 

 species (except for occasional abnormities;, and it is remarkable 

 that certain types of male sexual organs are generally connected 

 with certain type of hooks ; this is chiefly the case in the third, 

 fourth and fifth group of Faxon, while it is not in the first or sec- 

 ond, where similar types of sexual organs may be connected with 

 different types of hooks. 



If we consider that the presence of two pairs of hooks is cer- 

 tainly a more highly advanced stage than that of only one pair, that 

 is to say, that the difference of the number of hooks is only a differ- 

 ence in the degree of development of one and the same feature, it 



