12 WAR & STRUGGLE FOR EXISTENCE 



learning to trace whole branches, as yet separate at 

 their lower ends, but in themselves shapely and 

 showing a general resemblance to one another in 

 their progress from simple to complex. In his voyage 

 on the Rattlesnake he began to study jelly-fish, 

 probably chiefly because these were abundant in the 

 tropical seas. He made many dissections and 

 drawings, but instead of following the example of his 

 predecessors and contenting himself with stating 

 matters of detail concerning particular genera and 

 species, he tried to give " a broad and general view 

 of the whole class, considered as organized upon a 

 given type," and to enquire into its relations with 

 other classes. Having in this way arrived at a con- 

 ception of the peculiar organization of the group, 

 he hunted through the numberless fragile and flower- 

 like polyps of the sea and of fresh water, picked out 

 from them all those that revealed structure of the 

 medusa-type, and associated them in the great divi- 

 sion that we know now as Coelentera. He went even 

 further, and making use of von Baer's conception 

 that the younger stages of animals were more alike 

 than the later stages, he showed that there was an 

 essential similarity between the structure of Coelen- 

 tera and a stage passed through in the embryonic 

 history of vertebrate animals. By like methods and 

 with the same purpose, he brought together the 

 hitherto scattered creatures that we now know re- 

 spectively as Mollusca and as Ascidians, and traced 

 the unity of organization underlying the progressive 

 modifications of each group. And, finally, in his 

 lectures on cells and protoplasm, he showed that 

 animals and plants were composed of similar 



