ABSTRACT
The main point which prevented investigators, like Cuvier and his disciples, from accepting a genetic connection between the present and previous animal and plant forms of different appearance, was the conviction that the organisms always retained their specific peculiarities. As in ‘evolution,’ the alterations can certainly hardly be directly considered, but they show that the organism is no stubborn unchangeable form. A very fruitful domain for historical evolutionary hypotheses is presented by the manifold relations of various species of animals and plants to each other or of animals to plants. The deeper the transformation of the entire organism may be which it needs in order to become adapted to its new mode of life, the greater will its embryonic evolution differ from the earlier one. The embryogeny of the Mammalia is precisely as different from the embryogeny of the Fishes as a completely formed mammal is from a complete fish.
