452 POPULAR SCIENCE MONTHLY 



influences could bring about a directly responsive organic change, 

 which he assumed was inherited. Lamarck developed the well-known 

 view, previously advocated by Erasmus Darwin, that indirect responses 

 to the environment could be fixed in inheritance as so-called " acquired 

 characters," meaning by this phrase that such characters are acquisi- 

 tions during the life-time of an individual as the effects of disuse or 

 unusual use, or of new habits. Coming again to Darwin, we find that 

 he endeavored to support Lamarck's doctrine and to supplement his 

 doctrine of selection by adding the theory of pangenesis. According 

 to this every cell of every tissue and organ of the body produces minute 

 particles called gemmules, which partake of the characters of the cells 

 that produce them. The gemmules were supposed to be transported 

 throughout the entire body, and to congregate in the germ-cells, which 

 would be in a sense minute editions of the body which bears them, and 

 would so be capable of producing the same kind of a body. If true, 

 this view would lead to the acceptance of Lamarck's or even Buffon's 

 doctrine, for changes induced in any organ by other than congenital 

 factors could be impressed upon the germ-cell, and would then be trans- 

 ported together with the original specific characters to future genera- 

 tions. Darwin was indeed a good Lamarckian. 



But the researches of post-Darwinians, and especially those of the 

 students of cellular phenomena, have demonstrated that such a view 

 has no real basis in fact. Many naturalists, like Naegeli and Wiesner, 

 were convinced that there was a specific substance concerned with 

 hereditary qualities as in a larger way protoplasm is the physical basis 

 of life. It remained for Weismann to identify this theoretical sub- 

 stance with a specific part of the cell, namely, the deeply-staining sub- 

 stance, or chromatin, contained in the nucleus of every cell. Bringing 

 together the accumulating observations of the numerous cytologists of 

 his time, and utilizing them for the development of his somewhat specu- 

 lative theories, Weismann published in 1882 a volume called " The 

 Germ-Plasm," which is an immortal foundation for the later work on 

 inheritance. The essential principles of the germ-plasm theory are 

 somewhat as follows : The chromatin of the nucleus contains the deter- 

 minants of hereditary qualities. In reproduction, the male sex-cell, 

 which is scarcely more than a minute mass of chromatin provided with 

 a thin coat of protoplasm and a motile organ, fuses with the egg, and 

 the nuclei of the two cells unite to form a double body, which contains 

 equal contributions of chromatin from the two parental organisms. 

 This gives the physical basis for paternal inheritance as well as for 

 maternal inheritance, and it shows why they may be of the same or 

 equivalent degree. When, now, the egg divides, at the first and later 

 cleavages, the chromatin masses or chromosomes contained in the double 

 nucleus are split lengthwise and the twin portions separate to go into 



