TRANSACTIONS OF SECTION D. 677 



and alternation of generations. All three naturalists, Chamisso, Sars and Steen- 

 strup, laid stress on this point. In an insect, they would have said, there is de- 

 velopment with metamorphosis. The same animal passes from larva to pupa, and 

 from pupa to imago. In Aurelia or Salpa, however, the animal which lays eggs 

 is not the animal which buds, but its progeny. The cycle of the life-history 

 includes two generations and many individuals. 



This view has spread very widely, and if we were to judge by what is com- 

 monly taught, 1 think that we should recognise this as the doctrine now prevalent. 

 It is however, in my opinion, far inferior as an explanation of the facts to that 

 adopted by Leuckart, Carpenter and Huxley, who regard the whole cycle, from 

 egg to egg, as one life-history. Huxley and Carpenter, differing in this from 

 Leuckart, do not shrink from calling the whole product of the egg an animal, 

 €ven though it consists of a multitude of creatures which move about and seek 

 their food in complete independence of one another. Rather than ignore the unity 

 of the life-history of Aurelia or Salpa, they would adopt the most paradoxical 

 language. This attitude was forced upon them by the comparative method. They 

 I'efused to study Aurelia, for example, as an animal apart ; it had its near and its 

 remoter relatives. Among these is the fresh-water Hydra, which develops with- 

 out transformation, buds off other Hydras when food is plentiful, and at length 

 becomes sexually mature. Budding is here a mere episode, which may be brought 

 in or left out, according to circumstances. The same individual polyp which buds 

 iifterwards produces eggs. The life-history of Salpa cannot be traced with equal 

 facility to a simple beginning, for it presents points of difficulty, on which the 

 learned differ. In the Polychtet Worms, however, we find a beautiful gradation 

 leading up to alternation of generations. We begin with gradual addition of new 

 segments and increasing specialisation of the two ends of the body, the fore end 

 becoming non-reproductive, and the hinder end reproductive. Then we reach a 

 stage (Syllis) in which the reproductive half breaks off from the fore part, and 

 forms (after separation) a new head, while the fore part adds new segments behind. 

 In Autolytus the new head forms before separation, and many worms may cohere 

 for a time, forming a long chain with heads at intervals. In Myrianida the worms 

 break up first, and afterwards become sexually mature. We should gather from 

 these cases that alternation of generations may arise by the introduction of a 

 budding-stage into a development with transformation. The polyp or worm buds 

 while young and lays eggs at a later time. The separation of the two processes of 

 reproduction often becomes complete, each being restricted to its own place in the 

 life-history. As a rule the worm or polyp will bud while its structure is uncom- 

 plicated by reproductive organs. It is easy to propagate some plants by cutting 

 one of the leaves into sections, and making every section root itself, and grow into 

 a new plant ; but we can seldom do the same thing with a flower. There may 

 therefore be a distinct advantage to particular animals and plants in dividing the 

 life-history into two stages, an earlier budding, and a later egg-laying stage. 



The advantage to be drawn from budding is easily seen in those animals which 

 find it hard to gain access to a favourable site. Thus a Taenia ^ is very lucky when 

 it establishes itself in the intestine. Once there, it goes on budding indefinitely. 

 It is harder to trace the advantage in the case of many polyps, though some 

 (Cunina, &c.) admit of the same explanation as Taenia. There are yet other cases 

 (some Worms, Salpse, &c.) in which our ignorance of the conditions of life renders 

 a satisfactory explanation impossible at present. 



The budded forms often differ in structure from the budding forms which 

 produce them, and many writers and teachers make this difference part of tho 

 •definition of alternation of generations. I think that Leuckart has suggested a 

 probable explanation in his essay of 1851,* which is still thoroughly profitable 



' This case is quoted by Leuckart. 



- 'Ueber Metamorphose, ungeschlechtliclie Vermehrung, Generationswechsel,' 

 &its.f. 7viss. Zool., Bd. III. Equally important is the same author's treatise, Ueher 

 den Poh/morjjMsmus der Individtcen oder die Ersclieinung der Arheitstheilung in der 

 JVatur, Giessen, 1851. 



