Facts and Factors of Development 



A. PHENOMENA OF DEVELOPMENT 



Ontogeny and Phytogeny. One of the greatest and most far- 

 reaching themes which has ever occupied the minds 'of men is 

 the problem of development. Whether it be the development of 

 an animal from an egg, of a race or species from a pre-existing 

 one, or of the body, mind and institutions of man, this problem 

 is everywhere much the same in fundamental principles, and 

 knowledge gained in one of these fields must be of value in each 

 of the others. Ontogeny and phylogeny are not wholly distinct 

 phenomena, but are only two aspects of the one general process 

 of organic development. The evolution of races and of species 

 is sufficiently rare and unfamiliar to attract much attention and 

 serious thought; while the development of an individual is a 

 phenomenon of such universal occurrence that it is taken as a 

 matter of course by most people, something so evident that it 

 seems to require no explanation ; but familiarity with the fact of 

 development does not remove the mystery which lies back of it, 

 though it may make plain many of the processes concerned. The 

 development of a human being, of a personality, from a germ 

 cell is the climax of all wonders, greater even than that involved 

 in the evolution of a species or in the making of a world. 



The fact of development is everywhere apparent; its principal 

 steps or stages are known for thousands of animals and plants; 

 even the precise manner of development and its factors or causes 

 are being successfully explored. Let us briefly review some of 

 the principal events in the development of animals, and particu- 

 larly of man, and then consider some of the chief factors and 

 processes of development. Most of our knowledge in this field 

 is based upon a study of the development of animals below man, 

 but enough is now known of human development to show that in 

 all essential respects it resembles that of other animals, and that 

 the problems of heredity and differentiation are fundamentally 

 the same in man as in other animals. 



