The intermediate variety, consequently, will exist in lesser numbers from inhabiting a narrow and lesser area; and practically, as far as I can make out, this rule holds good with varieties in a state of nature.

I have met with striking instances of the rule in the case of varieties intermediate between well-marked varieties in the genus Balanus.

And it would appear from information given me by Mr.Watson, Dr.Asa Gray, and Mr.Wollaston, that generally, when varieties intermediate between two other forms occur, they are much rarer numerically than the forms which they connect.

Now, if we may trust these facts and inferences, and conclude that varieties linking two other varieties together generally have existed in lesser numbers than the forms which they connect, then we can understand why intermediate varieties should not endure for very long periods: why, as a general rule, they should be exterminated and disappear, sooner than the forms which they originally linked together.
For any form existing in lesser numbers would, as already remarked, run a greater chance of being exterminated than one existing in large numbers; and in this particular case the intermediate form would be eminently liable to the inroads of closely allied forms existing on both sides of it.

But it is a far more important consideration, that during the process of further modification, by which two varieties are supposed to be converted and perfected into two distinct species, the two which exist in larger numbers, from inhabiting larger areas, will have a great advantage over the intermediate variety, which exists in smaller numbers in a narrow and intermediate zone.