If we see isolated species, we may presume the absence of some more closely connected, and which have not yet been discovered.

Already there are genera, and even entire orders, nay, whole classes which present this state of things." He then goes on to present, "as a guide to conjecture," what his successors now assert as a fact: "In the first place, if we examine the whole series of known animals, from one extremity to the other, when they are arranged in the order of their natural relations, we find that we may pass progressively, or at least with very few interruptions, from beings of more simple to those of more compound structure; and in proportion as the complexity of their organization increases, the number and dignity of their faculties increase also.

Among plants a similar approximation to a graduated scale of being is apparent.

Secondly, it appears, from geological observations, that plants and animals of more simple organization existed on the globe before the appearance of those of more compound structure, and the latter were successively formed at more modern periods, each new race being more fully developed than the most perfect of the preceding one."[22] From this gradation of nature, thus stated, the evolutionists go on to infer genealogy, the birth descent of the larger from the smaller, and of the more complex from the simpler forms, as the only scientific explanation.

But it is by no means the only scientific explanation of the order of nature.