But, in practice, the formation of specific groups is more or less qualified by considerations based upon what is known respecting variation. It is a matter of observation that progeny are never exactly like their parents, but present small and inconstant differences from them. Hence, when specific identity is predicated of a group of individuals, the meaning conveyed is not that they are all exactly alike, but only that their differences are so {292} small, and so inconstant, that they lie within the probable limits of individual variation. Observation further acquaints us with the fact, that, sometimes, an individual member of a species may exhibit a more or less marked variation, which is propagated through all the offspring of that individual, and may even become intensified in them. And, in this manner, a _variety_, or _race_, is generated within the species; which variety, or race, if nothing were known respecting its origin, might have every claim to be regarded as a separate morphological species. The distinctive characters, of a race, however, are rarely equally well marked in all the members of the race. Thus suppose the species A to develop the race A + _x_; then the difference _x_ is apt to be much less in some individuals than in others; so that, in a large suite of specimens, the interval between A + _x_ and A will be filled up by a series of forms in which _x_ gradually diminishes. Finally, it is a matter of observation that modification of the physical conditions under which a species lives favours the development of varieties and races. Hence, in the case of two specimens having respectively the characters A and A + _n_, although, _primâ facie_, they are of distinct species; yet if a large collection shows us that the interval between A and A + _n_ is filled up by forms of A having traces of _n_, and forms of A + _n_ in which _n_ becomes less and less, then it will be {293} concluded that A and A + _n_ are races of one species and not separate species. And this conclusion will be fortified if A and A + _n_ occupy different stations in the same geographical area. Even when no transitional forms between A and A + _n_ are discoverable, if _n_ is a small and unimportant difference, such as of average size, colour, or ornamentation, it may be fairly held that A and A + _n_ are mere varieties; inasmuch as experience proves that such variations may take place comparatively suddenly; or the intermediate forms may have died out and thus the evidence of variation may have been effaced. From what has been said it follows that the groups termed morphological species are provisional arrangements, expressive simply of the present state of our knowledge.