678 • EEPORT— 1897. 



reading. He attributes the peculiarities of the larva mainly to tlie circumstance 

 that it is turned out at an early age to shift for itself. In the budded forms there 

 is no such necessity. The parent has established itself on a good site which com- 

 mands a sufficiency of food. Until it has done this, it does not bud at all. 

 The young which it produces asexually need not disperse in infancy, at least until 

 crowding sets in. The tradesman who has foimded a business puts his elder boys 

 into the shop ; perhaps the younger ones may be obliged to try their luck in a 

 distant town. The budded forms, reared at the cost of the parent, may therefore 

 omit the early larval stages at least, and go on at once to a later or even to the 

 final stage. Thus the head of Taenia, when it has fixed itself in the intestine, pro- 

 duces sexual segments ; the redia of Distomum produces cercariaj or more redise, 

 omitting the locomotive embrj'o ; the scyphistoma produces ephyrte. The saving 

 of time must often be great, and the days saved are days of harvest. Think how 

 much a tree would lose if in the height of summer it were unable to bud, and 

 could only propagate by seeds. If the budded forms are sexual, while the budding- 

 forms are not, there is an obvious explanation of the difterence in form. Even 

 where there is no such fundamental difterence in function, the circumstances of 

 early life are very different, and may well produce an unhkeness upon which 

 Natural Selection may found a division of labour. 



No one who tries to trace origins can rest satisfied with Steenstrup's account 

 of alternation of generations. He makes no effort to show 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 alterna- 

 tion of generations. 



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

 over the word generation. For my own part I believe that such words as genera- 

 tion, individiml, organ, larva, adult cannot 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- 

 appropriate 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 develop- 

 ment with transformation, but not alternation of generations. 



It is, I think, of importance to be able to resolve so peculiar a phenomenon as 

 alternation of generations into processes which are known to occur separately, and 

 which may have arisen imperceptibly, becoming gradually emphasised by the 

 steady action of the conditions of life. Every startling novelty that can thus be 

 explained extends the application of that principle 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 development with 

 transformation and also of budding. They yield also the most admirable examples 

 of division of labour. We have Ilydrozoan 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 Siphonophora the colony 

 becomes pelagic, and floats at the surface of the sea. Then the medusae no longer 



• This 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-defined alternation of 

 generations, but no metamorphosis. 



^ Cf. Leuckart, loo. cit., p. 183. 



