LESS COMPETITION, WITH ESCAPE FROM ENEMIES. IOI 
will be produced in any case in which the degree of segregation and 
the ratios of fertility for the pure and crossed breeds are known. 
Taking formula (4), reached a little below (on page 105), it will 
* be seen that if J = m, that is, if cross-breeds are as fertile as pure- 
breeds, the ratios of cross-breeds to pure-breeds increases with each 
generation, and in time the pure-breeds will be overwhelmed. 
14. Segregate Freedom from Competition and Segregate Escape from Enemies. 
Before leaving the subject of segregate survival it may be well to 
give an illustration of the effects of these two forms of the principle 
when codperating with industrial and social segregation. On certain 
of the South Pacific islands there are found two species of rats occupy- 
ing the same areas; but one species has strong legs and claws fitted 
for climbing the trees which they inhabit, while the other species lives 
on the ground and has limbs fitted forits separate sphere olf activities. 
As the latter is the prevalent type on surrounding groups of islands, 
there seems to be good reason for believing that the tree species was 
derived from the ground species. 
Let us now consider some of the conditions that might easily arise 
in the formation of such a species. Let us first suppose that there 
arises a period of scarcity in which the ordinary food of the species is 
obtained with difficulty, while on the trees are to be found either 
fruits which are more or less accessible to a good climber, or perhaps 
snails, which in tropical regions live constantly on the trees of the 
dense forests. Again, let us suppose that the ordinary rat of the orig- 
inal stock finds his strength sufficient to enable him to spend only 
one-tenth of each day seeking food in the trees, and that accordingly 
he can do better by remaining on the ground; but there are a very 
few individuals, say one in a thousand, who are able to spend one- 
quarter of their time in climbing; and for them there is abundance of 
food, which induces them to spend most of their time in the trees. 
These few find holes in the trees, where they congregate during hours 
of rest, and thus become more or less segregated from the rest of the 
species. 
Again, let us assume that half of these rats feeding in the trees are 
of variation a, with instincts leading them to spend one-half of their 
time in the trees and one-half of their time on the ground, the result 
being that one-half of them form cross unions and have their nests on 
the ground, while one-half of them pair with tree-feeding rats and 
have their nests on the trees. Further, let us assume that the fertility 
of the pure-breeds and half-breeds is the same, each pair producing 
about 80 young during their life. Therefore, the multiplier for the 
