of their hybrid-offspring should generally correspond, though due to distinct causes; for both depend on the amount of difference of some kind between the species which are crossed.  Nor is it surprising that the facility of effecting a first cross, the fertility of the hybrids produced, and the capacity of being grafted together—­though this latter capacity evidently depends on widely different circumstances—­should all run, to a certain extent, parallel with the systematic affinity of the forms which are subjected to experiment; for systematic affinity attempts to express all kinds of resemblance between all species.

First crosses between forms known to be varieties, or sufficiently alike to be considered as varieties, and their mongrel offspring, are very generally, but not quite universally, fertile.  Nor is this nearly general and perfect fertility surprising, when we remember how liable we are to argue in a circle with respect to varieties in a state of nature; and when we remember that the greater number of varieties have been produced under domestication by the selection of mere external differences, and not of differences in the reproductive system.  In all other respects, excluding fertility, there is a close general resemblance between hybrids and mongrels.  Finally, then, the facts briefly given in this chapter do not seem to me opposed to, but even rather to support the view, that there is no fundamental distinction between species and varieties.

CHAPTER 9.  ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.

On the absence of intermediate varieties at the present day.  On the nature of extinct intermediate varieties; on their number.  On the vast lapse of time, as inferred from the rate of deposition and of denudation.  On the poorness of our palaeontological collections.  On the intermittence of geological formations.  On the absence of intermediate varieties in any one formation.  On the sudden appearance of groups of species.  On their sudden appearance in the lowest known fossiliferous strata.

In the sixth chapter I enumerated the chief objections which might be justly urged against the views maintained in this volume.  Most of them have now been discussed.  One, namely the distinctness of specific forms, and their not being blended together by innumerable transitional links, is a very obvious difficulty.  I assigned reasons why such links do not commonly occur at the present day, under the circumstances apparently most favourable for their presence, namely on an extensive and continuous area with graduated physical conditions.  I endeavoured to show, that the life of each species depends in a more important manner on the presence of other already defined organic forms, than on climate; and, therefore, that the really governing conditions of life do not graduate away quite insensibly like heat or moisture.  I endeavoured, also, to show that intermediate varieties, from existing in lesser