REVIEW — THE NATURALIST’S LIBRARY. 
705 
themselves, to a certain extent, to a change of external circum- 
stances, the extent varying greatly according to the species. But 
we shall find as we proceed that these mutations, thus superin- 
duced, are not the effect of chance, as the author would lead us to 
suppose. The law of variety is not blindly or indiscriminately 
followed, but, in every instance, is intimately related to the natural 
wants and propensities of the species. 
The true distinction generally considered between different 
species, is their incapacity of propagating with each other an 
offspring capable again of continuing itself by subsequent propa- 
gations. But our author is of opinion that we are not justified in 
assuming this as law, as it has been repeatedly proved that two 
species very nearly allied to each other will produce a hybrid 
offspring, and that the hybrid is again productive with an indivi- 
dual of the true specie of either parent. 
Mr. John Hunter says, alluding to this subject, “ that if it be 
true that the mule has been known to breed, and which must be 
allowed to be an extraordinary fact, yet from the copulation of 
mules being frequent, and the circumstance of their breeding very 
rare, he should rather attribute it to a degree of monstrosity in the 
organs of the mule which conceived, as not being a mixture of 
two different species, but merely those of either the male or the 
female. And this is not a far-fetched argument either, when we 
consider that some true species produce monsters with a mixture 
of both sexes, and that many animals of distinct sex are incapable 
of breeding at all.” 
This opinion of Mr. Hunter was formed from experiments made 
with the wolf, the dog, and the jackal. The dog, he found, would 
breed with the wolf and the jackal ; and the mule in each case 
•would breed again with the dog. 
But be it observed, that in these experiments, as well in all 
others that we have heard of, there was always one parent at least 
of pure breed, and no proof was obtained that a true hybrid could 
be perpetuated ; a fact of which no examples are yet recorded, 
either in regard to a mixture of the horse and ass, or any other of 
the mammalia. 
The author appears to consider that it was in some such manner 
— perhaps from a cross with the Hemionus Asinus (a very supe- 
rior kind of ass) — that the Isabella breed of horses and the 
Eelback dun-breed of Scotland was first established, having 
asinine streaks on the back, and cross bar-streaks on the joints and 
shoulders. We would here remark that, if it could be shewn that 
this breed of horses could not possibly have been produced in a 
more natural way, there would be some reason for agreeing with 
the author; but there is another wav in which this might be 
