BROODS AND RACES OF THE PERIODICAL CICADA. 231 
I consider this case as of equal interest with the previous one; for it 
is an example of complete isolation between the forms of one species 
through diversity in their instincts. Whether these divergent in- 
stincts are sexual or social may be a matter of question; but in either 
case they are effectual in preventing crossing. 
If future investigation shows that the small form is often produced 
directly from the eggs of the large form, it will have but little claim to 
be regarded as a separate race; but even then, if the small form breeds 
only with its own kind, as has been reported by several observers, and 
if the offspring persistently reproduce the characters of the parents, 
it will have to be considered something more than a dimorphic form 
of the large one. It would, in this case, be a dimorphic form that is 
assuming the character of a species. If the two forms were without 
segregative sexual and social instincts, then, with cross-fertility, the 
small form would be rapidly absorbed by the large form, which greatly 
preponderates in numbers; and with cross-sterility the small form 
would rapidly become extinct; for, through the comparative scarcity 
of their numbers, the representatives of the small form would have 
but little chance of mating with each other. 
On the other hand, if the sexual and social isolation is complete, 
it matters but little whether the forms are mutually sterile, for the sep- 
arate races or species will be protected by the positive segregation 
produced by the divergent instincts, even if the negative segregation 
depending on structural incompatibility and lack of physiological 
adaptation is entirely wanting. It is only when associated with pos- 
itive segregation that is partial in its results that negative forms of 
segregation become important factors in the preservation of diverging 
forms. - 
In animals that pair, isolation through sexual and social instincts 
plays a similar role in giving preémptive power to the males of a given 
species over the females of the same species that is played by poten- 
tial and prepotential isolation in organisms whose fertilizing elements 
are distributed by wind and water. In the one case instinctive 
and in the other potential segregation, arising between varieties of 
the same species, marks these varieties as being the initial forms 
of divergent species. 
This species presents another form of isolation which is of much in- 
terest, though it has not yet resulted in forms that can be ranked as 
different races. I refer to the complete cyclical segregation that exists 
between different broods of a given race appearing in different years. 
Of the thirteen-year race there are seven broods and of the seven- 
teen-yearracefourteen. Asan example of different broods occurring 
