216 THE USE OF THE MICROSCOPE 



Thus the cell theory gave a vision of the marvels of 

 ontogeny, but the problem of the origin of different cell 

 classes (apparently) still waits for its Darwin to provide a 

 first approach to a solution. 



In ontogeny we are usually certain that each individual 

 comes from one cell, and the indisputable sequence of 

 changes from one kind of embryo to another in the course 

 of ontogeny is only to be equalled in importance by phylo- 

 geny when dealing with the experimental change of kinds 

 of beings, such as chromosomal mutations and gene 

 mutations. 



The Chromosome Theory of Inheritance. — The second 

 great microscopical discovery is, the writer thinks, the 

 chromosome theorj^ of inheritance. Before the chromo- 

 somes could be successfully brought into such a theory, it 

 was, of course, necessary to know what inheritance was; 

 and, for a long time, no one but Mendel had the insight and 

 patience to find this out. The material for the chromosome 

 theory was sufficient, when Mendel's results were redis- 

 covered, to allow of the enunciation of a promising working 

 hypothesis; though natural conservatism seems to have 

 delayed its general adoption until more evidence had 

 accumulated in its favor. 



The chromosomes afford not only a theory of inheritance, 

 but the basis for a theory of life. For all, or most of the 

 living structures, such as cilia, muscle fibers, nerve threads, 

 and gland cells are products, so far as we know, of the 

 chromosomes. Hence, a theory of life is a theory of the 

 chromosomes. 



One thing, as already noted, the chromosomes do not 

 apparently afford yet, and that is a satisfactory working 

 hypothesis of ontogeny, to say nothing of a theory. 



Visibility of Genes. — It remains {inter alia) for the micro- 

 scope to complete the chromosome theory by the thorough 

 investigation of the genes. The counting, and especially 

 the identification, of these would perhaps give support 

 to some hypothesis of ontogeny. For if genes are found to 

 be lost (by non-division at any stage), the non-reversibility 



