466 GROWTH OF BIOLOGY IN THE NINETEENTH CENTURY. 



they arrange themselves in a sequence of complicated figures, and so 

 distribute themselves to two "daughter" organisms. 



But the most impressive argunuMit for the doctrine that the cell itself 

 uuist, in its turn, be a highly complex organism is, above all, the part 

 it plays in the developmental process of higher plants and animals, 

 for the cells of co;g and seed, as Niigi'li has explained in a philosoph- 

 ical manner, are the vehicles of the luunberless properties by which the 

 different species of organisms are distinguished. They therefore 

 consist of hereditary masses or idioplasm, which, in order to include 

 the inherited properties which are destined to become manifest in 

 growth, must be a highly organized l)od3'. 



With this, 1 come to the third great adA'ance which biology has 

 effected in the nineteenth century. More than previous ages, our cen- 

 tury has been dominated by the idea of development. It has made 

 itself felt as a working leaven in many departments of knowledge, phi- 

 losophy, history, philology, sociology, and geology, but nowhere more 

 than in biolog}'. Yet the living organism is the only natural object 

 which puts before our eyes, in a short space of time, a complete cycle 

 of development, from the fecundated egg to the perfect creation again 

 productive of new life. 



On closer examination the question of the development of the 

 organism embraces two different (juestions: First, that of the develop- 

 ment of the individual — that is, the cycle of phenomena through which 

 it rims, starting from the egg until its natural death; and, secondly, 

 the ([ucstion as to how so extraordinarily complicated a product as we 

 have found the vegetable or animal organism to be arose in a natural 

 way in the course of the earth's histor3^ Ontogeny and phylogeny, 

 to avail ourselves of a pair of terms introduced l)y Haeckel, are the 

 two fields of research into which the doctrine of development of 

 organisms is divided. 



Ontogeny alone is subject to direct scientific investigation. Frorc 

 the fertilized egg on it is possible, by the choice of suitable plants and 

 animals, to follow their development step by step from one stage to 

 the next. Here again the microscope has been the instrument with 

 the help of which we have i>enetrated deeply into ontogeny and have 

 set forth the universal laws of formation. Since the days of Pander 

 and Carl Ernst von Baer, who has been called the " father of the his- 

 tory of development" on account of his immortal services, thanks to 

 a great roll of German, French, English, Russian, and Italian embry- 

 ologists, there has been erected a comprehensive, excellent, lordly, 

 well-joined, S3\stematic history of development. In details many proc- 

 esses have yet. to be examined more nicely; but, on the whole, the 

 essence of individual development has been explained upon its mor- 

 phological side, and we have a right to be proud of the insight into it 

 which we have gained, especially when we recall how the greatest men 



