1912^ Zoology in America 65 



not experimentallj demonstrate or disprove the theory in its 

 entirety. We then ask if any of the facts of observation and 

 experiment contradict the theory, or do they corroborate it in 

 that they prove explicable by it. And are there many such facts 

 and of diverse kinds ? In other words, have we worked with the 

 theory a long time and found it to hold good? If so, after a 

 time we practically cease to question it directly, and it comes 

 into a use that is habitual and almost reflex. So it is with the 

 theory of universal gravitation and so it is with the theory of 

 organic evolution. 



A gulf separates Agassiz's theory from that of evolution, 

 and yet we must recognize that the actual investigations of the 

 earlier school, the solid discovery of demonstrable facts and 

 their formulation into generalizations, went on along much the 

 same lines as in the later period. We may therefore with justice 

 say that in America from about 1850 to 1890 it was the interest 

 in fundamental form that dominated zoology. This was the 

 great period of morphology to which zoology owes so much, 

 and which began in Europe in the early years of the nineteenth 

 century. Many of the most substantial results of biological 

 inquiry have resulted from this intense comparative study of 

 structure, adult and embryonic. That the body of all but the 

 simplest animals, the protozoa, is composed of tissues, and these 

 of microscopic units, the cells, each of which is comparable with 

 a protozoan ; that the egg from which a metazoan animal starts is 

 but a single cell, and that this by division produces many cells 

 which differentiate into the nerve, muscle, gland, and other 

 tissues ; that the cells early become arranged into two primary 

 layers, which as such make up the body of low metazoa (coe- 

 lenterates), but which in higher forms become infolded and out- 

 folded so as to give rise to many internal organs : these are fun- 

 damental discoveries which, as Oscar Hertwig has said, are the 

 answers to questions that baffled the most acute biologists and 

 philosophers of earlier ages. 



The bulk of the discoveries of morphologists are, of course, 

 such as require for their comprehension some technical train- 

 ing. All I can say here is that this wealth of knowledge, so 



