154 CAUSES OF VARIATION 



units required to explain the function and achievements of the 

 germ plasm, the bit of vitalized matter that holds within its 

 substance all the potentialities of its particular kind of life. 



Summary. The causes of variation are closely connected 

 with the mechanism of growth and differentiation. The cell is 

 the unit of structure and all growth is by cell division ; but it 

 is not the unit of differentiation of different parts of the body, 

 because all parts arise from one original cell, the germ cell. 



Cell division seems admirably adapted to insure absolute trans- 

 mission without variability of any kind. But both differentiation 

 and variability are facts. We seek, therefore, a " physiological 

 unit" more minute than the cell, whose activities and possibly 

 whose combinations with other physiological units of different 

 properties are able to bring forth first differentiation within the 

 body and later differences between different individuals. 



ADDITIONAL REFERENCES 



CHROMOSOME VESICLES IN MATURATION. By W. M. Smallwood. Science, 



XXI, 386. 

 CYTOLOGICAL FEATURES OF FERTILIZATION. By W. H. Blackman. 



Proceedings of the Royal Society, London, LXIII, 400-401. 

 FERTILITY OF EGGS AFTER REMOVAL OF COCK. By L. G. Jarvis. 



Experiment Station Record, XI, 671. 

 LAWS OF EMBRYONIC DEVELOPMENT: THE LAW OF VON BAER. By 



Otto Glaser. Science, XV, 976-982. 

 MECHANISM OF DEVELOPMENT. By William Turner, F.R.S. Popular 



Science Monthly, LVII, 561-575. 

 ONTOGENETIC AND PHYLOGENETIC VARIATION. By H. F. Osborn. 



Science, IV, 786-789. 



PROBLEM OF DEVELOPMENT. By E. B. Wilson. Science, XXI, 281-293. 

 PROTOPLASMIC STRUCTURE. By E. B. Wilson. Science, II, 893-899; 



X, 33-45- 



SOME OBSERVATIONS AND CONSIDERATIONS UPON MATURATION PHE- 

 NOMENA OF GERM CELLS. By T. H. Montgomery. Biological 



Bulletin, VI, 1904. 

 STRUCTURE AND FORMATION OF Pus CELLS. Experiment Station 



Record, XIV, 1016. 

 VITALITY OF POLLEN. (Roses, twenty-two days; clivias, three months.) 



Experiment Station Record, XIII, 620; (Bear, thirty days) XV, 872. 



