"_The series which constitutes the animal scale resides in the distribution of the groups, and not in that of the individuals and species._ "I have already said[166] that by this shaded graduation in the complication of structure I do not mean to speak of the existence of a linear and regular series of species or even genera: such a series does not exist. But I speak of a quite regularly graduated series in the principal groups, _i.e._, in the principal system of organizations known, which give rise to classes and to great families, series most assuredly existing both among animals and plants, although in the consideration of genera, and especially in that of species, it offers many lateral ramifications whose extremities are truly isolated points. "However, although there has been denied, in a very modern work, the existence in the animal kingdom of a single series, natural and at the same time graduated, in the composition of the organization of beings which it comprehends, series in truth necessarily formed of groups subordinated to each other as regards structure and not of isolated species or genera, I ask where is the well-informed naturalist who would now present a different order in the arrangement of the twelve classes of the animal kingdom of which I have just given an account? "I have already stated what I think of this view, which has seemed sublime to some moderns, and indorsed by _Professor Hermann_." Each distinct group or mass of forms has, he says, its peculiar system of essential organs, but each organ considered by itself does not follow as regular a course in its degradations (modifications). "Indeed, the least important organs, or those least essential to life, are not always in relation to each other in their improvement or their degradation; and an organ which in one species is atrophied may be very perfect in another. These irregular variations in the perfecting and in the degradation of non-essential organs are due to the fact that these organs are oftener than the others submitted to the influences of external circumstances, and give rise to a diversity of species so considerable and so singularly ordered that instead of being able to arrange them, like the groups, in a single simple linear series under the form of a regular graduated scale, these very species often form around the groups of which they are part lateral ramifications, the extremities of which offer points truly isolated. "There is needed, in order to change each internal system of organization, a combination of more influential circumstances, and of more prolonged duration than to alter and modify the external organs.