1899] MEHNERT' S PRINCIPLES OF DEVELOPMENT 389 



induced by new relations to the external conditions of life. In other 

 words, he regards the growth of an organ as the result of two factors. 

 One of these consists of the inherited qualities, — the constitutional 

 energies. The other (epigenetic) factor is in a word environmental. 



The primary, constitutional, or inherited factors (Evolutionsdeter- 

 mination) are fixed when the germ-cell is separated from the parent 

 which bears it, or when fertilisation has occurred. The secondary, 

 functional, or acquired factors (Entwickelungsdetermination) are found 

 in the environmental conditions and in the internal relations of parts. 

 Many have made this theoretical distinction before, the difficulty is to 

 apply it. 



It is easy to show that the course of development may be modified 

 by external circumstances and by the immediate organic environment, and 

 the author recalls many more or less familiar instances of determination 

 by nutrition, by temperature, by actual functioning, by association with 

 other parts, and by the environment in the broadest sense. But it 

 seems to us that the conception of environmental relation requires more 

 careful analysis than the author gives it ; thus we have to distinguish 

 (a) the relation of normal functional dependence, (b) the relation which 

 results in a " modification " in the strict sense, i.e. when the limit of 

 organic elasticity is transcended, and (c) the relation in which the 

 environment supplies a variation-stimulus or inhibits a variation- 

 tendency. 



In regard to heredity, it seems to Mehnert that if a correlation be 

 admitted between the rate of embryonic development and the functional 

 importance of an organ, we are forced to admit the inheritance of 

 acquired characters, to doubt which seems to him almost a reproach to 

 the intelligence of a physician or observant naturalist ! He insists on 

 the parallelism between the racial evolution of an organ and its growth 

 in the development of the individual, and seems to suggest that the 

 former explains the latter. 



But the peculiarity, the idiosyncrasy, of Mehnert's view is, that 

 there is a transmission of specific energies from the parent to the germ- 

 plasm which is the foundation of the offspring. The body of the 

 parent is to the germ-cell as a mother-magnet to a daughter-magnet. 

 There is in inheritance, he conceives, a constant element, the material 

 germ-plasm ; but there is also a variable factor, namely, this mysterious 

 energy on the intensity of which the rate of development depends. 

 In this hypothesis he does not mean to postulate an imponderable vital 

 force ; he heads his work with Ostwald's axiom that everything is to 

 be interpreted in terms of energy. 



If each step in racial evolution means not a mere lopping off or 

 tacking on, but an alteration in the developmental rhythm of the 

 organism, which seems a most reasonable conception, then the talk one 

 sometimes overhears about uniformity of development had better be 

 silenced, and already many only whisper the old error. Mehnert shows 



27 NAT. SC. VOL. XIV. NO. 87. 



