For those who do not cherish pedantic and sectarian prejudices, this hypothesis is changed into assurance by modern discoveries; it is shown in the transformations and transitions of paleontological forms; in the embryogenic evolution of so many animals, man included, which, according to their various species, reveals the lower types whence they issued; in the successive forms taken by the foetus; in the powerful and indisputable laws of selection; in the modifications by adaptation of the different organisms, and in the effects of isolation.  This is the only rational explanation, confirmed as it is by fresh facts every day, of the multiplicity and variety of organic forms in the lapse of time; unless, indeed, we ascribe such variety to a miracle, even more difficult to accept than the difficulties of the opposite-theory.

I admit that evidence for the complete demonstration of this theory is sometimes wanting; the gaps between the fossil fauna and flora and those of modern times are neither few nor unimportant; but on the other hand, such proofs are accumulating, and the gaps are filled up every day, so that we may almost assert that in some way or other, by means somewhat different from those on which we now rely, the great rational principle of evolution will be successfully and permanently established.

It is more than twenty years since, in ways and by study peculiar to ourselves, we first devoted ourselves to this theory, and while we gave a conscientious consideration to opposite theories, so as to estimate with sincerity their importance and value, we could not relinquish our conviction that every advance in physical, biological, and social science served to confirm the theory of evolution.

It must not be supposed that I make any dogmatic assertion, which might possibly be erroneous, when I say that the evidence of facts does not contradict the assumptions of modern science.  Sincere convictions should offend no one, nor do they indicate an a priori conflict with other beliefs.  Every one is justified in thinking his own thoughts when he speaks with moderation and supports his peculiar opinions with a certain amount of learning.

It is not denied, even by those who oppose modern theories respecting the genesis of organisms, that there are, excluding some psychical elements, many and important points of resemblance between man and animals in the exercise of their consciousness, intelligence, and emotions, if indeed they are not identically the same.  The comparative psychology of man and animals plainly shows that the perceptions, both in their respective organs and in their mode of action, act in the same way, especially in the higher animals; and the origin, movements, and associations of the imagination and the emotions are likewise identical.  Nor will it be disputed that we find in animals implicit memory, judgment, and reasoning, the inductions and deductions from one special fact to another, the passions, the physiological language of gestures, expressive of internal emotions, and even, in the case of gregarious animals, the combined action to effect certain purposes; so that, as far as their higher orders are concerned, animals may be regarded as a simple and undeveloped form of man, while man, by his later psychical and organic evolution, has become a developed and complex animal.[4]