5IO THE GENERAL PRINCIPLES OF ZOOLOGY 





available for study with the very high magnifying powers of 

 the microscope. 



One of the most important results of embryological research 

 has been the correction of many errors of classification, for 

 often the real relationship of animals does not appear until 

 their development is known. Thus the studies of the eminent 

 Russian embryologist, Kowalewsky, showed the Tunicata to 

 belong to the Chordata instead of to the Molluscoidea, where they 

 had previously been classified. Many organisms once regarded 

 as distinct species have since been shown to be merely larval 

 stages of other organisms, and finally our knowledge of the 

 minute structure of protoplasm and its various modifications in 

 cells is due largely to improvements in histological technique 

 within the last decade or two of the century. 



For the trend of research at the present day, we owe much to 

 the works of Weismann and of Mendel. Weismann's theorv of 

 the germ-plasm has proved the greatest stimulus to the study 

 of experimental embryology and the phenomena of fertilization. 

 Mendel's law of the purity of the germ-cells has given the first 

 real foundation for the scientific study of heredity, and it is 

 being elaborated by extensive observations and experiments in 

 breeding. As a direct result of the extended researches in 

 embryology, there has arisen a phenomenal interest in the 

 structure and activities of the cell, and many investigators are 

 devoting their attention exclusively to this line of work; so 

 greatly has our knowledge increased en this subject that it is 

 now recognized as a distinct branch of biology, under the name 

 of Cytology, We should note, too, the more precise methods 

 which are being introduced into the study of variation, a subject 

 of much interest through its bearing on theoretical biology; 

 variations are now studied statisticallv or quantitatively, the 

 extent of variation in some particular characteristic or group of 

 characteristics being measured, and the results tabulated and 

 represented graphically by curves, with mathematical precision. 



The second half of the nineteenth century likewise saw great 

 advances in palaeontology, the most notable being due to the 

 researches of Marsh and Cope in .America. Our knowledge of 

 deep-sea life has been accumulated during this period; many 

 European countries and the United States have sent out expedi* 



