Mr. Darwin says :— 
“It would appear that any change in the habits of life, whatever 
these habits may be, if great enough, tends to affect in an inexplicable 
manner the powers of reproduction.” Again he says: “ Those 
animals which usually breed freely under confinement, rarely breed, 
I was assured, in the Zoological Gardens within a year or two after 
their first importation.”—(The Variation, vol. z'z., pp. 159-60.) 
It is a large assumption, then, on the part of Mr. Wallace 
when he assumes that, all other conditions being favour 
able, immigrants will exhibit at once the greatest possible 
fertility. 
“ All animals are capable of multiplying so rapidly, that if a single 
pair were placed in a continent with abundance of food and no 
enemies, they might fully stock it in a very short time. Thus a bird 
which produces ten pairs of young during its life-time (and this is far 
below the fertility of some birds) will, if we take its life at five years 
increase to a hundred millions in about forty years—a number 
sufficient to stock a large country.”—(The Geographical Distribution 
of Animals, vol. z., p. io.) 
As a matter of fact, we know that immigrants into new 
localities exhibit for a time at least a limited fertility. 
For example,— 
“The fecundity of geese introduced upon the table-land of Bogota, 
according to Dr. Roulin, was much altered in the first season. They 
laid only a few eggs, the eggs were small in number—a fourth, at the 
most, were hatched ; and more than half of the young goslings died 
in the first month, but from generation to generation the fecundity 
tended to return.”—(P. Lucas, b Héredite Naturelle. vol. ii., pp. 
478-9-) 
In the case of a single pair, this limited fertility might well 
be fatal to any attempt at colonisation. 
There is one other special difficulty to be noted. Animals 
and plants, eggs or seeds carried to great distances would 
find themselves, in many cases, in conditions which varied 
considerably from those to which they were accustomed. 
Supposing that they were able to survive all the difficulties 
of transit and of establishing themselves, they would have