94 ORTMANN — AFFINITIES OF CAMBARUS. [April 13, 
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 Caméarus 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. Ina few cases the latter is very evident. 
Copulatory Hooks of the Male.—F¥axon 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 
