408 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



logical deformations should have made their appearance complete 

 as we see them. They evidently originated from normal forms. 

 Nowhere does the idea of evolution assert itself more positively. 

 When the zoologist is in the presence of such astonishing forms as 

 those which are shown him by the epicarid isopods, parasites on other 

 crustaceans, he is forced to believe — as the development of these 

 creatures proves — that originally they were normal isopods which 

 did not differ from the existing free forms. It is, therefore, only 

 after the perfection of the isopod type that the evolution of the 

 epicaridae took place under the influence of parasitism. This second- 

 ary evolution, less far away from us than the first, may be more ac- 

 cessible to us. Consequently the study of the parasitic forms has a 

 special interest in helping us to reach a knowledge of the laws of 

 evolution. At least these laws will perhaps appear more clearly to 

 us in this field. 



The material offered by the facts of parasitism, and by those of 

 commensalism or of symbiosis which are connected with them, is, as 

 you can imagine, immense, and the real difficulty is to choose among 

 these facts to keep oneself within the limits imposed by a course of 

 lectures. The harvest of new facts to be gathered is still large and 

 fruitful. We have reached a period in the history of the biological 

 sciences where we accurately measure the difficulty of the great 

 problems. During the heroic period of Darwinism it was generally 

 believed that embryology would reveal all the secrets of morphology. 

 To-day we know that it can not be thus. Morphology often asserts 

 its truth ; it does not actually explain itself, because we do not succeed 

 in seeing in it the exact result of a definite mechanism which might 

 if necessary be produced experimentally. On the other hand, the 

 great laws of morphology are established. The golden age was that 

 of Cuvier, Geoffroy Saint-Hilaire, and their immediate successors. 

 In the domain of physiology, on the contrary, which is that of the 

 existing factors and which ignores the past, the progress of all the 

 experimental sciences brings forward every day the possibility of 

 new investigations. Thus cellular physiology is to-day in full de- 

 velopment and in rapid progress. And we are therefore sometimes 

 tempted to say that morphology is finished, or nearly so, that in any 

 event we can not expect from it results which are worthy of extended 

 efforts. Therefore let the young generations turn away from it and 

 give themselves entirely to the physiological side of biology. 



I shall not deny that there is a certain amount of truth in this idea, 

 yet I believe that, formulated in a rather positive manner as it some- 

 times is by eminent biologists, it is a dangerous exaggeration. Doubt- 

 less many of the pillars of morphology and of descriptive zoology 

 are firmly set up and will not move. But it is an illusion to believe 

 for this reason that we really know morphology. The enormous 



