As a rule such deviations from the normal course may be classified as consequences of a prolonged residence of the animal germs in the egg or in the womb, the result of which is that owing to the presence of an abundant quantity of nourishing yolk, or through direct connection with the circulatory system of the mother, they in the early stages of their development are relieved of the necessity of acquiring nourishment through their own efforts, and therefore all the contrivances necessary to that purpose may be dispensed with. For this reason we find the primitive processes of development, as Professor Sollas has lately shown, most frequently preserved in marine animals which have never changed nor abandoned their element in the course of the history of their species, in the case of which, therefore, no occasion could ever have arisen for supplementary changes in the process of their development. Much more frequently do we meet with this change in the case of fresh-water animals, for often the rapid currents of their elements, for example a river, will not suffer these to leave the egg in any very helpless larval condition, and in addition fresh water is subject to other unfavorable changes, as the drying up of streams. Also the larvæ of carnivorous animals, which from the very beginning of independent life need more strength to acquire their means of existence, are so completely developed in the richly provisioned eggs in which they take their form, that they emerge therefrom in an almost perfected state of being, as, for example, young sharks and cephalopods. In this kind of animal life, as well as in the case of forms which are brought forth alive from the parent, although they see the light of day much later, comparatively, there takes place not only a great abbreviation of the first stages of existence in the entering upon a more direct path of development, but also changes occur in the form of the original designs because of the limitation of room due to the presence of yolk in the egg, the reason for which is easy to perceive. In many other cases the mechanical cause of the change in development can be directly recognised; for example, in the case of, the tree-toad of the Antilles (_Hylodes marticinensis_), which, owing to the absence of pools lasting through the dry season, is obliged of necessity to remain in the egg during its tadpole stage, that is to say, to skip this stage, as it were; for which reason the formation of external gills in its case is entirely omitted.