In modern times a theory of development was first suggested by Goethe in his _"Italienische Reise."_ Acting under the same mental urge for seeing diverse forms under a unifying principle, Goethe looked for the original form of plant life, the _Urpflanze_, the plant which would be at once simple enough to stand for a type of all plants and yet susceptible to variation in so many directions that all plants might derive from it their origin. Goethe has also clothed this conception in poetic form. The first philosophic statement of the hypothesis is found in Immanuel Kant's _"Kritik der Urteilskraft,"_ 1790. In paragraph 80 we find a discussion of the similarity between so many species of animals, not only in their bony structure, but also in the arrangement of their other parts, a similarity which, says Kant, "casts a ray of hope," that all forms may be traced back to original simple forms, to "a generation from a common ancestor," rising from the lowest forms to man, "according to mechanical laws." Kant assumes that, for instance, certain aquatic animals by and by formed into amphibia, and from these after some generations were produced land animals. A treatise of the same philosopher entitled _"Presumable Origin of Humanity"_ suggests that man in the early age of the world was developed from "mere animal creatures." Even a universal law of world-formation (cosmic evolution) was set forth by Kant in a work which he published anonymously in 1775. In its relations to animal life a development theory was first clearly set forth by Karl Ernst von Baer (died 1876). In his _"Entwickelungsgeschichte der Tiere"_ (1828), the author explains "Entwickelung" as a progress from simple to complex forms. He believes that in evolution there is a fundamental idea that "goes through all the forms of cosmic and animal development." A predecessor of von Baer had been the Frenchman, Lamarck. From von Baer, Herbert Spencer, about 1850, adopted the definition of evolution. The hypothesis entered a new phase through Charles Darwin's epochmaking work: _"The Origin of Species."_ The keynote of Darwin's theory is Natural Selection, by which term the development of all living forms is referred to the working of certain laws which in the reproduction of plants and animals preserved those individuals which were best fitted to survive the struggle for existence. The Darwinian theory may be summarized thus: The Darwinian Hypothesis. 1. Every kind of animal and plant tends to increase in numbers in a geometrical progression. 2. Every kind of animal and plant transmits a general likeness, with individual differences, to its offspring. 3. Past time has been practically infinite. 4. Every individual has to endure a very severe struggle for existence, owing to the tendency to geometrical increase of all kinds of animals and plants, while the total animal and vegetable population (man and his agency excepted) remains almost stationary.