Biology classes

Usage examples of genera.

We may instance Rubus, Rosa, and Hieracium amongst plants, several genera of insects, and several genera of Brachiopod shells.

Fresh-water and salt-loving plants have generally very wide ranges and are much diffused, but this seems to be connected with the nature of the stations inhabited by them, and has little or no relation to the size of the genera to which the species belong.

Moreover, on the view of the origin of genera which I shall presently give, we have no right to expect often to meet with generic differences in our domesticated productions.

I am inclined to suspect that we see in these polymorphic genera variations in points of structure which are of no service or disservice to the species, and which consequently have not been seized on and rendered definite by natural selection, as hereafter will be explained.

In most polymorphic genera some of the species have fixed and definite characters.

Under genera, including the most polymorphic forms, Mr Babington gives 251 species, whereas Mr Bentham gives only 112, -- a difference of 139 doubtful forms!

Moreover, the species of the large genera which present any varieties, invariably present a larger average number of varieties than do the species of the small genera.

Both these results follow when another division is made, and when all the smallest genera, with from only one to four species, are absolutely excluded from the tables.

Now Fries has remarked in regard to plants, and Westwood in regard to insects, that in large genera the amount of difference between the species is often exceedingly small.

In this respect, therefore, the species of the larger genera resemble varieties, more than do the species of the smaller genera.

Or the case may be put in another way, and it may be said, that in the larger genera, in which a number of varieties or incipient species greater than the average are now manufacturing, many of the species already manufactured still to a certain extent resemble varieties, for they differ from each other by a less than usual amount of difference.

In genera having more than the average number of species in any country, the species of these genera have more than the average number of varieties.

In large genera the species are apt to be closely, but unequally, allied together, forming little clusters round certain species.

In all these several respects the species of large genera present a strong analogy with varieties.

But by steps hereafter to be explained, the larger genera also tend to break up into smaller genera.