Incipient Species

Alph. De Candolle and others have shown that plants which have very wide ranges generally present varieties.

This might have been expected, as they become exposed to diverse physical conditions, and as they come into competition (which, as we shall hereafter see, is a far more important circumstance) with different sets of organic beings.

In any limited country, a species will have give rise to well-marked varieties if it:

• has the most population
• is the most widely diffused in that country

This is shown by my tables.

Hence, the dominant species:

• range widely over the world
• are the most diffused in their own country
• are the most populous

This produces well-marked varieties or incipient species.

For varieties to become permanent, they have to struggle with the other inhabitants of the country.

The dominant species will be the most likely to yield offspring which will still inherit those advantages that made their parents dominant.

If the plants inhabiting a country and described in any Flora are divided into 2 equal masses, all those in the larger genera being placed on one side, and all those in the smaller genera on the other side, a somewhat larger number of the very common and much diffused or dominant species will be found on the side of the larger genera.

This, again, might have been anticipated.

The mere fact of many species of the same genus inhabiting any country, shows that there is something in the organic or inorganic conditions of that country favourable to the genus;

Consequently, we might have expected to have found in the larger genera, or those including many species, a large proportional number of dominant species.

But so many causes tend to obscure this result, that I am surprised that my tables show even a small majority on the side of the larger genera.

I will here allude to only two causes of obscurity.

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.

Again, plants low in the scale of organisation are generally much more widely diffused than plants higher in the scale; and here again there is no close relation to the size of the genera.

The cause of lowly-organised plants ranging widely will be discussed in our chapter on geographical distribution.

From looking at species as only strongly-marked and well-defined varieties, I was led to anticipate that the species of the larger genera in each country would oftener present varieties, than the species of the smaller genera;

Wherever many closely related species (i.e. species of the same genus) have been formed, many varieties or incipient species ought, as a general rule, to be now forming. Where many large trees grow, we expect to find saplings.

Where many species of a genus have been formed through variation, circumstances have been favourable for variation; and hence we might expect that the circumstances would generally be still favourable to variation.

On the other hand, if we look at each species as a special act of creation, there is no apparent reason why more varieties should occur in a group having many species, than in one having few.

To test this, I have arranged the plants of 12 countries and the coleopterous insects of 2 districts into 2 nearly equal masses.

The species of the larger genera on one side, and those of the smaller genera on the other side.

A larger proportion of the species on the side of the larger genera present varieties, than on the side of the smaller genera.

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.

These facts are of plain signification on the view that species are only strongly marked and permanent varieties.

Whenever many species of the same genus have been formed, or where, if we may use the expression, the manufactory of species has been active, we ought generally to find the manufactory still in action, more especially as we have every reason to believe the process of manufacturing new species to be a slow one.

This is the case, if varieties be looked at as incipient species; for my tables clearly show as a general rule that, wherever many species of a genus have been formed, the species of that genus present a number of varieties, that is of incipient species, beyond the average.

It is not that all large genera are now varying much, and are thus increasing in the number of their species, or that no small genera are now varying and increasing.

If this had been so, it would have been fatal to my theory.

Geology tells us that:

• small genera have in time often increased greatly in size
• large genera have often come to their maxima, declined, and disappeared.

Where many species of a genus have been formed, on an average many are still forming.

There are other relations between the species of large genera and their recorded varieties which deserve notice.

There is no infallible criterion by which to distinguish species and well-marked varieties.

In those cases in which intermediate links have not been found between doubtful forms, naturalists are compelled to come to a determination by the amount of difference between them, judging by analogy whether or not the amount suffices to raise one or both to the rank of species.

Hence the amount of difference is one very important criterion in settling whether two forms should be ranked as species or varieties.

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.

I have tested this numerically by averages.

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.

Moreover, the species of the large genera are related to each other, in the same manner as the varieties of any one species are related to each other.

No naturalist pretends that all the species of a genus are equally distinct from each other; they may generally be divided into sub-genera, or sections, or lesser groups.

Fries has well remarked, little groups of species are generally clustered like satellites around certain other species. And what are varieties but groups of forms, unequally related to each other, and clustered round certain forms–that is, round their parent- species?