On reading The Origin of Species

p. 110

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 haveing many species, than in one having few.

In other words, and turning the statement on its head somewhat, where the environment (and/or other factors) have allowed an organism to diversify and we find many different varieties, we are more likely to find a greater variety of allied species of the organism than in the opposite case where the environmental factors have not been favourable to the diversification.

Darwin goes on to make the case for this statement with real-world examples.