It seems to me more probable, however, that they do store in the south, but not in the north; for all the difficulties which attend the preservation of the seed in the granaries in the south would be greatly increased in the wet climates of Northern Europe, and there, moreover, the greater cold would render the ants torpid almost throughout the winter, when food would not be required. But the question is plainly an open one. We may also ask why it is that only a very few out of the many species of ants which inhabit the shores of the Mediterranean should possess this habit of collecting seeds, and differ so widely in their manner of living, from their neighbours? If we wish to put ourselves in the way to answer these queries, the first thing we should do would be to examine and compare the structure of the digestive organs and parts of the mouth in harvesters and non-harvesters, with a view to seeing whether there may not be some capital difference here. These observations demand some skill in dissection and preparation, and in regretting that it has not been in my power to make them, I can only hope that some one more skilled than I am may undertake the subject. It seems probable, that in warmer latitudes there are many conditions which favour the rapid increase of ants, so that a given tract of country in Southern Europe, for example, must have on an average more colonies to support than a similar tract in the north, and that to meet this increase of population, it has therefore become necessary for these creatures to seek their subsistence from as many and as dissimilar sources as possible. The fierce conflicts over booty both between rival nests of the same and of distinct species, tend to show that, even as things are, they frequently have to fight for their food. Hitherto, as far as I have been able to learn, only nineteen true harvesting ants have been detected in the whole world, limiting this term to those species which make the collection of seeds the principal occupation of their outdoor lives, and are evidently in the main dependent upon this kind of food for subsistence. Now if we compare these nineteen species of ants[40] together a very curious fact forces itself upon our notice--namely, that all of them are closely related, so much so that not only do all belong to the same division of ants (the tribe _Myrmicineæ_), but that with one exception (_Pseudomyrma_) all would have been placed by the great Fabricius in one genus, _Atta_, and the one exception is not far removed from it. [Footnote 40: These are _Myrmica (Atta) barbata_, from Texas and Mexico; _Œcodoma (Atta) cephalotes_, from Brazil and Mexico; _Œcodoma (Atta) providens_, from India; _Œcodoma (Atta) diffusa_, from India; _Atta rufa_, from India; _Pheidole (Atta) megacephala_, from South France; _Atta barbara_, from South France, Capri, and Algiers; _Atta structor_, from South France; and _Pseudomyrma rufo nigra_, from India.]