The distinction of species he explains by the destruction of intermediate graduated forms.

"Thus living plants and animals are not separated from the extinct by new creations, but are to be regarded as their descendants through continued reproduction." A well-known French botanist, M.Lecoq, writes in 1854 ("Etudes sur Geograph." Bot.tom.i, page 250), "On voit que nos recherches sur la fixite ou la variation de l'espece, nous conduisent directement aux idees emises par deux hommes justement celebres, Geoffroy Saint-Hilaire et Goethe." Some other passages scattered through M.Lecoq's large work make it a little doubtful how far he extends his views on the modification of species.
The "Philosophy of Creation" has been treated in a masterly manner by the Rev.Baden Powell, in his "Essays on the Unity of Worlds", 1855.
Nothing can be more striking than the manner in which he shows that the introduction of new species is "a regular, not a casual phenomenon," or, as Sir John Herschel expresses it, "a natural in contradistinction to a miraculous process." The third volume of the "Journal of the Linnean Society" contains papers, read July 1, 1858, by Mr.Wallace and myself, in which, as stated in the introductory remarks to this volume, the theory of Natural Selection is promulgated by Mr.Wallace with admirable force and clearness.
Von Baer, toward whom all zoologists feel so profound a respect, expressed about the year 1859 (see Prof.Rudolph Wagner, "Zoologisch-Anthropologische Untersuchungen", 1861, s.  51) his conviction, chiefly grounded on the laws of geographical distribution, that forms now perfectly distinct have descended from a single parent-form.
In June, 1859, Professor Huxley gave a lecture before the Royal Institution on the "Persistent Types of Animal Life".

Referring to such cases, he remarks, "It is difficult to comprehend the meaning of such facts as these, if we suppose that each species of animal and plant, or each great type of organisation, was formed and placed upon the surface of the globe at long intervals by a distinct act of creative power; and it is well to recollect that such an assumption is as unsupported by tradition or revelation as it is opposed to the general analogy of nature.

If, on the other hand, we view "Persistent Types" in relation to that hypothesis which supposes the species living at any time to be the result of the gradual modification of pre-existing species, a hypothesis which, though unproven, and sadly damaged by some of its supporters, is yet the only one to which physiology lends any countenance; their existence would seem to show that the amount of modification which living beings have undergone during geological time is but very small in relation to the whole series of changes which they have suffered." In December, 1859, Dr.Hooker published his "Introduction to the Australian Flora".

In the first part of this great work he admits the truth of the descent and modification of species, and supports this doctrine by many original observations.
The first edition of this work was published on November 24, 1859, and the second edition on January 7, 1860..