CHAP, xiv THE CONTINUANCE OF LIFE 299 



standing why each part should be able to regrow the 

 whole, for each is a fair sample of the original unity. 

 Indeed, when a large Protozoon is cut into two or three 

 pieces with a knife, each piece, if nucleated, may 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 

 proceed 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 small buds are sometimes set adrift ; 

 the Hydra forms daughter polypes by budding, and these 

 are set free ; sea-anemones and several worms, and 

 even some starfishes, multiply by the separation of 

 comparatively large pieces. But this mode of multi- 

 plication which is called asexual has evident limita- 

 tions. 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 exception 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 separation of special reproductive cells ; 



