430 A CENTURY OF SCIENCE 



it is in some animals not irrevocably fixed, and the normal 

 effect of the sex chromosomes may be inhibited by 

 abnormal conditions in the developing embryo, as is 

 demonstrated by the recent work of Lillie and others. 



The cytological basis for Mendelian inheritance has 

 been very extensively studied by Morgan and his pupils 

 in connection with their work on inheritance in the com- 

 mon fruit fly Drosophila. The evidence supports Weis- 

 mann's earlier hypothesis that the chromosomes are the 

 bearers of the heritable factors, and that these are 

 arranged in a series in the different chromosomes. This 

 theory is shown to be in such strict accord with both the 

 cytological studies and the results of experimental breed- 

 ing that Morgan has ventured to indicate definite points 

 in particular chromosomes as the loci of definite heri- 

 table factors, or genes. 



Confirmation of this view is furnished by the behavior 

 of the so-called sex-linked characters, the genes for which 

 are situated in the same chromosome as that which 

 carries the sex factor. Many ingenious breeding experi- 

 ments indicate further that all the hereditary characters 

 in Drosophila are borne in four great linkage groups 

 corresponding with the four pairs of chromosomes which 

 the cells of this fly possess. 



Comparative Physiology. 



None of the experimental fields has been of greater 

 importance in zoological progress than that which con- 

 cerns the functions of the various organs. Without this 

 companion science morphology and comparative anatomy 

 would have become unintelligible. American investiga- 

 tors, among whom G. H. Parker stands prominent, have 

 taken a leading part in this field also. 



Neurology. The physiological analysis of the com- 

 ponents of the nervous system, both in vertebrates and 

 invertebrates, is another important branch of experimen- 

 tal biology. The 28 volumes of the Journal of Compara- 

 tive Neurology attest the large influence that American 

 investigators have had in the development of this science. 



Regeneration. Experimental studies on the powers 

 of regeneration in plants and animals have been made 

 from the earliest times. During the past few years, how- 



