Immanuel Kant, one of the most influential philosophers of the modern era, also presaged the feasibility of an evolutionary theory: The agreement of so many kinds of animals in a certain common structure, which seems to be fundamental not only in their skeletons, but also in the arrangement of the other parts - so that a wonderfully simple typical form, by the shortening and lengthening of some parts, and by the suppression and development of others, might be able to produce an immense variety of species - allows a ray of hope, however faint, to enter our minds, that here perhaps some result may be obtained, by the application of the principle of the mechanism of nature (without which there can be no natural science in general). This analogy of forms, which with all their differences seem to have been produced in accordance with a common prototype, strengthens our suspicions of an actual blood-relationship between them in their derivation from a common parent through the gradual approximation of one class of animals to another - beginning with the one in which the principle of purposiveness seems to be best authenticated, ie. man, and extending down to the polyps, and from these even down to mosses and lichens, and arriving finally at raw matter, the lowest stage of nature observable by us.
If [man] is not to stifle his human feelings, he must practise kindness towards animals, for he who is cruel to animals becomes hard also in his dealings with men. We can judge the heart of a man by his treatment of animals.