LIFE HISTORY STUDIES OF ANIMALS. 499 



account of alternation of generations. He makes no effort to sTiow how 

 it came about. Instead of considering alternation of generations as a 

 peculiar case of development with metamorphosis, complicated by 

 asexual reproduction/ he considers asexual reproduction as a peculiar 

 case of alternation of generations.^ He ignores all the facts which show 

 that the alternation may have been gradually attained, an omission 

 which is only excusable when we note that his treatise is dated 1842. 

 He asserts dogmatically that there is no transition from metamorphosis 

 to alternation of generations. 



It is impossible to think much on this subject without falling into 

 difficulties over the word generation. For my own part I believe that 

 such words as generation, individual, organ, larva, adult can not be 

 used quite consistently in dealing with a long series of animals whose 

 life histories vary gradually and without end. Ordinary language, 

 which was devised to meet the familiar and comparatively simple course 

 of development of man and the domestic animals, is not always appro- 

 priate to lower forms, with complex and unusual histories. If we are 

 resolved at all hazards to make our language precise and uniform, we 

 either fall into contradictions, or else use words in unnatural senses. 



Certain recent discussions render it necessary to point out that there 

 can be no alternation of generations without increase by budding. If 

 a single larva produces a single sexual animal, as when a pluteus 

 changes to an echinus, there is development with transformation, but 

 not alternation of generations. 



It is, I think, of importance to be able to resolve so peculiar a phe- 

 nomenon as alternation of generations into processes which are known 

 to occur separately, and which may have arisen imperceptibly, becoming 

 gradually emphasized by the steady action of the conditions of life. 

 Every startling novelty that can thus be explained extends the applica- 

 tion of that prin(;iple which underlies the theory of natural selection — 

 I mean the principle that a small force acting steadily through a long- 

 time may produce changes of almost any magnitude. 



The hydrozoa yield innumerable and varied examples of develop- 

 ment with transformation and also of budding. They yield also the 

 most admirable examples of division of labor. We have hydrozoan 

 colonies, such as a budding Hydra, in which all the members are pretty 

 much alike, but we soon advance to differentiation of the feeding and 

 the reproductive members. In the siphouophora the colony becomes 

 pelagic, and floats at the surface of the sea. Then the medus.B no 

 longer break off and swim away, but are harnessed to the colony, and 

 drag it along. The colony may contain feeding j)olyps, which procure 

 and digest food for the rest; swimming bells, which are attached 

 medusa? ; perhaps afloat, which is a peculiar kind of swimming bell; 



iThis is a convenient short account of alternation of generations, but it will not 

 apply to every case. In Hydra, for instance, there is an ill-delined alternation of 

 generations, but no metamorphosis. 



^Cf. Leuckart, loc. cit., page 183. 



