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 still be 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 the truth of this anticipation I have arranged the plants of twelve countries, and the coleopterous insects of two districts, into two nearly equal masses, the species of the larger genera on one side, and those of the smaller genera on the other side, and it has invariably proved to be the case that a larger proportion of the species on the side of the larger genera presented 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 least genera, with from only one to four species, are altogether excluded from the tables.