INDIVIDUAL DEVELOPMENT 249 



It is not at all certain that it will be possible to reduce all the 

 problems of the physiology of development to such categories as we 

 have mentioned. The subject is full of unsolved problems, but so far 

 as I can see no one has shown any real reason for assuming ultra- 

 physical agencies in any of the events, and there is the same pragmatic 

 reason for refusing to assent to such suggestions, which are made all 

 too frequently, that there is in other fields of science. If we will be 

 consistent, we are driven to the conclusion that the apparent simplicity 

 of the germ is real, that the germ contains no gemmules, or determinants 

 or other representative particles; that development is truly epigenetic, 

 a natural series of events that succeed one another according to physico- 

 chemical and physiological laws; the explanation of the sequence con- 

 sists simply in the discovery of each of its steps. 



Applications 



The problems of heredity and variation are included in a true 

 physiological conception of the individual development; but some bio- 

 logical conceptions that have more or less status and reputation are in- 

 consistent with it. Such are the inheritance of acquired characters, 

 atavism, and the theory of unit^charactera. The first is a familiar 

 problem that I shall not argue anew; the second logically implies the 

 presence of ancestral representative particles in the germ, which is in- 

 consistent with a physiological theory of development. But it is obvious 

 that the facts united under the name of atavism or reversion take their 

 place naturally in a physiological theory of development, as arrests of 

 development, or modification of environment, or in other ways. 



The theory of unit characters deserves more attention for it is 

 essentially a modern theory, and counts numerous adherents. This, 

 conception has been most sharply formulated by De Vries in his Muta- 

 tionstheorie. He says: 



The properties of the organism are constructed of units which are sharply 

 distinguished from one another. These units may be united in groups, and in 

 related species the same units and groups occur. Intermediates between the 

 units, such as the external forms of plants and animals exhibit so abundantly, 

 are not found any more than between the molecules of chemistry. 



Bateson's allelomorphs constitute a similar conception. Such 

 hypothetical elements of organization must be conceived as distinct 

 from the germ on. They can be shuffled about from one generation 

 to another, and can, therefore, be introduced, removed or replaced in 

 the germ cells. 



It must be admitted that these conceptions fit certain facts of 

 inheritance in many hybrids fairly well, but the progress of discovery 

 has made necessary the installation of subsidiary hypotheses, so that 

 the most recent conceptions of unit characters are becoming extremely 



