14 GENERAL PRINCIPLES OF ZOOLOGY 



(essentially the Zoophyta of older zoologists) ; to the rest, in which the 

 alimentary canal and the body-cavity occur as two separate cavities, he 

 gave the name Echinoderma. 



Thus there resulted seven classes: Protozoa, Ccelentera, Echinoderma, 

 Vermes, Arthropoda, Mollusca, and Vertebrata. Still this arrangement 

 does not meet the requirements of a natural system and is more or less 

 unsatisfactory. Upon anatomical and embryological characters the 

 Brachiopoda, the Bryozoa, and the Tunicata have been separated from 

 the Mollusca; they form the subject of diverse opinions. The relation- 

 ships of the first two groups have not yet been settled: of the Tunicata 

 we know indeed that they are related to the Vertebrata, but the differences 

 are such that they cannot be included in that group. The only way out 

 of the difficulty is to unite vertebrates, tunicates, and some other forms in 

 a larger division, Chordata. The Vermes, too, must be divided, as will 

 appear in the .second part of this volume. 



In the last decade of the nineteenth century and the beginning of the 

 present, physiological investigation has taken a place beside morphology. 

 Its most important tool is experiment. While experiments have long 

 been invoked to settle biological problems, they are now used in the most 

 extensive and systematic manner; especially are methodical breeding and 

 crossing experiments employed to solve the problems of variation and 

 heredity. There are also investigations into the laws which regulate the 

 animal form, in which the separate stages of embryonic and post-embry- 

 onic development are exposed to modifying influences (removal or trans- 

 plantation of blastomeres or parts of the body, employment of different 

 temperatures, chemical, mechanical, electrical stimuli), and the results 

 are compared with those of normal conditions. An important aid in 

 these studies is the mathematical statistical method by which the value 

 of the results of observation and experiment is tested. The second half 

 of the century just closed was especially characterized by the development 

 of the theory of evolution, the history of which is given in a separate 

 section. 



HISTORY OF THE THEORY OF EVOLUTION. 



The theory of evolution has developed into a question whose impor- 

 tance might, on a superficial examination, be underrated, but which has 

 grown into a problem completely dominating zoological research, and 

 has occupied not only zoologists, but all interested in science generally. 

 This is the question of the logical value of the conceptions species, genus, 

 family, etc. 



