Sex as an Adaptation 447 
be extremely rash at present to make a generalization of this 
kind, for there are many forms known in which the only 
method of propagation that exists is the non-sexual one. In 
other words, there are no grounds for the assumption that 
this is a necessary condition for all kinds of protoplasm, but 
only for certain kinds. 
In the insects, crustaceans, rotifers, and in some plants there 
are a few species whose egg develops without fertilization. 
This makes it appear probable that the particular kind of 
protoplasm of these animals does not absolutely require 
union from time to time with the protoplasm of another 
individual having a somewhat different constitution. 
There is also an interesting parallel between the effects of 
solutions on the protozoans in Calkins’s experiments and cer- 
tain results that have been obtained in artificial partheno- 
genesis. It has been stated, that by brushing the unfertilized 
eggs of the silkworm moth a larger percentage will develop 
parthenogenetically ; and more recently it has been shown by 
Matthews that by agitation of the water in which the un- 
fertilized eggs of the starfish have been placed many of 
them will begin their development. It was first shown by 
Richard Hertwig that by putting the unfertilized eggs of the 
sea-urchin in strychnine solutions, they will begin to segment, 
and I obtained the samé results much better by placing the 
eggs.in solutions of magnesium chloride. Loeb then suc- 
ceeded in carrying the development to a later stage by using 
a different strength of the same solution, as well as by other 
solutions. Under the most favorable circumstances some of 
the eggs may produce larve that seem normal in all respects, 
but whether they can develop into adult sea-urchins has not 
yet been shown. 
These results indicate that one at least of the factors of 
fertilization is the stimulus given to the egg. On the other 
hand, the lack of vigor shown by many eggs that have been 
artificially fertilized indicates that some other result is also 
