As the study of living beings in their adult condition shows us that the differences between those which are constructed upon the same morphological type depend upon the degree to which specialisation of function is carried, so the study of development teaches us that the changes undergone by any animal in passing from the embryonic to the mature condition are due to the same cause. All the members of any given sub-kingdom, when examined in their earliest embryonic condition, are found to present the same fundamental characters. As development proceeds, however, they diverge from one another with greater or less rapidity, until the adults ultimately become more or less different, the range of possible modification being apparently almost illimitable. The differences are due to the different degrees of specialisation of function necessary to perfect the adult; and therefore, as Von Baer put it, the progress of development is from the general to the special.
It is upon a misconception of the true import of this law that the theory arose, that every animal in its development passed through a series of stages in which it resembles, in turn, the different inferior members of the animal scale. With regard to man, standing at the top of the whole animal kingdom, this theory has been expressed as follows: "Human organogenesis is a transitory comparative anatomy, as, in its turn, comparative anatomy is a fixed and permanent state of the organogenesis of man" (Serres). In other words, the embryo of a Vertebrate animal was believed to pass through a series of changes corresponding respectively to the permanent types of the lower sub-kingdoms - namely, the Protozoa, Coelen-terata, Echinodermata, Annulosa, and Mollusca - before finally assuming the true vertebrate characters. Such, however, is not truly the case. The ovum of every animal is from the first impressed with the power of developing in one direction only, and very early exhibits the fundamental characters proper to its sub-kingdom, never presenting the structural peculiarities belonging to any other morphological type. Nevertheless, the differences which subsist between the members of each sub-kingdom in their adult condition are truly referable to the degree to which development proceeds, the place of each individual in his own sub-kingdom being regulated by the stage at which development is arrested. Thus, many cases are known in which the younger stages of a given animal represent the permanent adult condition of an animal somewhat lower in the scale. To give a single example, the young Gasteropod (amongst the Mollusca) transiently presents all the essential characters which permanently distinguish the adult Pteropod. The development of the Gasteropod, however, proceeds beyond this point, and the adult is much more highly specialised than is the adult Pteropod.
The theory of development held by the supporters of the doctrine of Evolution is best expressed in the words of Prof. Haeckel. According to this eminent naturalist, "Ontogenesis" (or the development of the individual) "is the brief and rapid recapitulation of phylogenesis" (or the development of the species) "governed by the physiological functions of transmission (reproduction) and nutrition (adaptation). The organic individual, during the rapid and brief course of its individual development, repeats the most important of those changes of form which its ancestors have passed through during the long and gradual course of their palaeontological development, in accordance with the laws of transmission and adaptation."