"There are also those who have been for a long time, and strongly, occupied with the determination of the species, and who have consulted rich collections, who can understand up to what point species, among living bodies, merge one into another (_fondent les unes dans les autres_), and who have been able to convince themselves, in the regions (_parties_) where we see isolated species, that this is only because there are wanting other species which are more nearly related, and which we have not yet collected. "I do not mean to say by this that the existing animals form a very simple series, one everywhere equally graduated; but I say that they form a branching series, irregularly graduated, and which has no discontinuity in its parts, or which at best has not always had, if it is true that it is to be found anywhere (_s'il est vrai qu'il s'en trouve quelque part_). It results from this that the species which terminates each branch of the general series holds a place at least on one side apart from the other allied species which intergrade with them. Behold this state of things, so well known, which I am now compelled to demonstrate. "I have no need (_besoin_) of any hypothesis or any supposition for this: I call to witness all observing naturalists. "Not only many genera, but entire orders, and some classes even, already present us with portions almost complete of the state of things which I have just indicated. "However, when in this case we have arranged the species in series, and they are all well placed according to their natural relations, if you select one of them, and it results in making a leap (_saut pardessus_) over to several others, you take another one of them a little less remote; these two species, placed in comparison, will then present the greatest differences from each other. It is thus that we had begun to regard most of the productions of nature which occur at our door. Then the generic and specific distinctions were very easy to establish. But now that our collections are very much richer, if you follow the series that I have cited above, from the species that you first chose up to that which you took in the second place, and which is very different from the first, you have passed from shade to shade without having remarked any differences worth noticing. "I ask what experienced zoölogist or botanist is there who has not thoroughly realized that which I have just explained to you?