1004 COMPARATIVE. HISTORICAL. 



of the nervi erigentes). The vasomotor nerves may be excited reflexly; also 

 through irritation of the sciatic. The internal os is especially rich in nerves. 



After birth the entire uterus is deprived of its mucosa (decidua) ; its inner 

 surface, therefore, is like a wound- surf ace, upon which a new membrane is formed, 

 with a secretion at first resembling an infusion of meat, later containing a larger 

 number of cells, and finally becoming mucoid (lochia). The thick muscular layer 

 of the uterus undergoes gradual reduction through partial fatty degeneration 

 of its fibers. Within the lumen of the large vessels an obliterating connective- 

 tissue hyperplasia begins from the intima, and in the course of several months 

 diminishes the lumen of the vessels or occludes them. The unstriated muscle- 

 fibers of the media undergo fatty degeneration. The relatively large blood- 

 spaces at the placental site are plugged by thrombi, and the latter are invaded 

 by connective-tissue from the walls of the vessel. 



After birth secretion of milk sets in, with a peculiar effect upon the vas- 

 cular nervous system (milk-fever?), an increased amount of blood being sent to 

 the mammary glands on the second or third day. The institution of the first 

 respiratory movements in the new-born is discussed on p. 755. 



COMPARATIVE. HISTORICAL. 



Embryology must not omit to take into consideration the general develop- 

 ment of the entire animal kingdom. The question "How have the innumerable 

 animal forms at present living originated?" has in part been answered by the 

 statement that all species have been created as such from the beginning, "every 

 form is an embodied idea of creation"; all species, further, remain as such without 

 alteration; the "constancy of species prevails." In opposition to this view, 

 held by Linnaeus, Cuvier, Agassiz, and others, Jean Lamarck in 1809 developed 

 the doctrine of ''the unity of the animal kingdom," embodying the old idea of 

 Empedocles, namely that all species have developed by variation from a few funda- 

 mental species, that originally only a few fundamental species of lower formation 

 existed, from which the new, numerous specie^ have evolved a view supported 

 also by Geoffroy St. Hilaire and Goethe. After a long interval this thought was de- 

 veloped in a particularly fruitful way by Charles Darwin (1859) . He supported his 

 ''monistic conception" of the animal kingdom by a description of the manner in 

 which gradual evolution of species can be explained. Among the creatures of the 

 earth there takes place, for the preservation of life, a struggle of all against all, and 

 from this "struggle for existence" only those will go forth victorious that are char- 

 acterized by particularly striking qualities. Such qualities: strength, speed, size, 

 color, fruitfulness, are, however, hereditary, and thus it is evident that, in this 

 manner, to a certain degree through natural selection, an uninterrupted process 

 of improvement and thereby a gradual variation in species takes place. In 

 addition, the creatures are capable, within certain limits, of adapting themselves 

 to their surroundings and the prevailing necessities of external influences. In 

 this way, certain organs may undergo a useful transformation, while inactive 

 parts can gradually undergo involution to rudimentary organs. The gradual 

 alteration of animal forms thus resulting through "natural selection" finds its 

 prototype in "artificial selection" among animals and plants. It is known, for 

 instance, that breeders of animals are able, in a relatively short time, to produce 

 variations in form that are much more considerable than those between two well- 

 characterized species of animals. Thus, the skull of a mastiff and that of an 

 Italian grayhound exhibit a much greater difference than that of a fox as com- 

 pared with that of a similar species of dog. As in the case of artificial selection, 

 however, there is observed a sudden reversion to an ancestral type, so also in the 

 development of natural species atavism may occur. Obviously the ease of varia- 

 tion is increased by the widespread distribution of a given species in different 

 climates, as in this way different influences become operative. Thus, the migra- 

 tion of organisms may gradually contribute to variations in species (M. Wagner's 

 law ot migration). Inheritance of mutilations does not occur. 



Without entering upon the .details in the development of the different varieties 

 ot animals, the biogenetic fundamental law may be briefly discussed. According 

 to this "the history of the individual (ontogeny) is a brief repetition of the history 

 ot the family (phylogeny) . " Applied especially to man, this law implies that the 

 separate stages in the development of the human embryo, for example its exist- 

 ence as a unicellular ovum, as a collection of cells after cleavage has been completed, 

 as a cellular vesicle (germinal vesicle), as a two-layered vesicle, as a being without 



