CHAP. XII The Life-History of Animals 185 



why each part should be able to legrow the whole, for each 

 is a fair sample of the original whole. Indeed, when a 

 large Protozoon is cut into two or three pieces with a knife, 

 each fragment is often able to retain the movements and 

 life of the intact organism. Among the Protozoa we find 

 some in which the multiplication looks like the rupture of a 

 cell which has become too large ; in others numerous buds 

 are set free from the surface ; in others one definitely-formed 

 bud (like an overflow of the living matter) is set free ; in 

 others the cell divides into two equal parts, after the 

 manner of most cells ; and numerous divisions may also 

 occur in rapid succession and within a cyst, that is, in 

 limited time and space, with the result that many " spores " 

 are formed. These modes of multiplication form a natural 

 series. 



In the many-celled animals multiplication may still pro- 

 ceed by the separation of parts ; indeed the essence of 

 reproduction always is the separation of part of an organ- 

 ism to form — or to help to form — a new life. Sponges bud 

 profusely, and pieces are sometimes set adrift ; the Hydra 

 forms daughter polypes by budding, and these are set free ; 

 sea-anemones and several worms, and perhaps even some 

 star-fishes, multiply by the separation of comparatively large 

 pieces. But this mode of multiplication — which is called 

 asexual — has evident limitations. It is an expensive way 

 of multiplying. It is possible only among comparatively 

 simple animals in which there is no very high degree of 

 differentiation and integration. For though cut-off pieces 

 of a sponge. Hydra, sea-anemone, or simple worm may 

 grow into adult animals, this is obviously not the case 

 with a lobster, a snail, or a fish. Thus with the excep- 

 tion of the degenerate Tunicates there is no budding 

 among Vertebrates, nor among Molluscs, nor among 

 Arthropods. 



The asexual process of liberating more or less large 

 parts, being expensive, and possible only in simpler animals, 

 is always either replaced or accompanied by another 

 method — that of sexual reproduction. The phrase " sexual 

 reproduction " covers several distinct facts : (a) the separa- 



