FOUR TYFES OF DEVELOPMENT. 57 



fcebrates, is essentially similar, notwithstanding the great 

 variety of outward forms. Baer, however, independently 

 and almost simultaneously, furnished proof that the four 

 groups develop from the egg by entirely different processes, 

 and further, that the order of the series of embryonic forms 

 in the course of evolution is from the very beginning 

 identical in all animals of the same type, but, on the other 

 hand, different- in those of different types. Up to that 

 time, in making a classification of the animal kingdom, an 

 endeavour had always been made to arrange all animals, from 

 the lowest to the highest, from the infusoria to man, in 

 a single connected series of forms; and the false idea had 

 always been maintained, that there was a single unbroken 

 gradation of development from the lowest animal to the 

 highest. Cuvier and Baer proved that this conception is 

 totally erroneous, and that, on the contrary, there are four 

 wholly distinct types of animals, which must be distin- 

 guished not only as to their anatomical structure, but also 

 as to their embryonic evolution. 



As a result of this discovery, Baer succeeded in estab- 

 lishing a very important law, which we shall name in his 

 honour Baer's Law, and which he expresses as follows : 

 " The evolution of an individual of a certain animal form 

 is determined by two conditions : firstly, by a continuous 

 perfection of the animal body by means of an increasing 

 histological and morphological differentiation, or an increas- 

 ing number and diversity of tissues and organic forms; 

 secondly, and at the same time, by the continual transition 

 from a more general form of the type to one more specific." 

 The degree of perfection of the animal body depends on 

 the greater or less amount of heterogeneity there is in its 



