HISTORY OF ZOOLOGY. 19 



kingdom consisted of several co-ordinated unities, the 

 types, which exist quite independently side by side, within 

 which again there are higher and lower forms. The posi- 

 tion of an animal is determined by two factors: in the first 

 place, by its conformity to a type, through the structural 

 plan which it represents ; in the second place, by its de- 

 gree of organization, through the stage to which it at- 

 tains within its type. 



Comparative Embryology. Evolution vs. Epigen- 

 esis. The same results which Cuvier reached by the 

 \vay of Comparative Anatomy were attained two dec- 

 ades later by C. E. von Baer by the aid of Embry- 

 ology. Embryology is the youngest branch of Zoology. 

 What real material for this Aristotle knew, what was 

 written by Fabricius ab Aquapendente and Malpighi upon 

 the embryology of the chick, did not rise above the 

 range of aphorisms, which were not of sufficient value to 

 make a science. The difficulties which here encompassed 

 observation, due to the delicacy and the minuteness of the 

 developmental stages, were ameliorated by the invention 

 of the microscope and microscopical technique. Further, 

 the prevailing philosophical conceptions placed hindrances 

 in the way; generally speaking, there was no belief in 

 Embryology in the present sense of the word ; each or- 

 ganism was thought to be laid down even from the begin- 

 ning complete in all its parts, and only needed growth to 

 unfold its organs (Evolutio*} ; either the spermatozoon 

 must be the young creature which in the store of food in 

 the egg found favorable conditions for growth, or the egg 

 represents the individual and was stimulated to the 

 " Evolutio " by the spermatozoon. In its further conse- 

 quence this theory led to the doctrine of inclusion, which 

 taught that in the ovary of Eve were included the germs 

 of all human beings who have lived or ever will live. 



Caspar Friedrich Wolff in 1759 combated this idea 

 with his " Theoria generationis " ; he sought to prove by 



* It should be carefully noted that this original meaning of "evolution" i? 

 quite different from that at present prevailing. 



