498 LIFE HISTORY STUDIES OF ANIMALS. 



its structure is un(5omplicated 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 tbe 

 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 favorable site. Thus a 

 Tcenia^ 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, etc.) 

 admit of the same explanation as Tcmiia. There are yet other cases 

 (some worms, salptE, etc.) 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 differ- 

 ence part of the definition of alternation of generations. I think that 

 Leuckart has suggested a probable explanation in his essay of 1851,^ 

 which is still thoroughly profitable reading. He attributes the pecul- 

 iarities of the larva mainly to the 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 sufticiency 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 

 founded 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, 

 produces sexual segments; the redia of jOtstomwm produces cercarise 

 or more redise, omitting the locomotive embryo; the scyphistoma pro- 

 duces ephyrai. 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 j)ropagate 

 by seeds. If the budded forms are sexual, while the budding forms are 

 not, there is an obvious explanation of the difference in form. Even 

 where there is no such fundamental difference in function, the circum- 

 stances of early life are very different, and may well produce an unlike- 

 ness upon which natural selection may found a division of labor. 



No one who tries to trac« origins can rest satisfied with Steenstrup's 



' This case is quoted by Leuckart. 



-Ueber Metamorpbose, ungescblecbtliche Vermebruug, Geuerationswecbsel; Zeits. 

 f. wiss. Zool., Bd. III. Equally important is the same author's treatise, Ueber den 

 Polymorpbismus der Individueu oder die Erscheinung der Arbeitstheilung in der 

 Natur, Giessen, 1851. 



