CONJUGATION IN THE CRAYFISH, CAMBARUS AFFINIS 263 
actual mechanical contact. There is as yet no evidence that the 
light or chemical substances from the male and female act dif- 
ferently upon the males. 
Crossing of species is not yet accomplished, though most of 
the phases of conjugation are carried on between species, in 
captivity. 
The organs which are accurately adjusted during conjugation 
are of different values; some are ordinary and some specialized 
limbs; some are mere spines of the shell; one is a special invagina- 
tion not represented in many other animals. Though some are 
old and some new they are all alike necessary at present for the 
meeting of sperm and egg. 
It is by no means clear how any offered method of evolution 
can account for the assemblage of such constituents into their 
present working individuality. 
