454 



LIFE : OUTLINES OF GENERAL BIOLOGY 



even up to Tunicates, it is attended with obvious difficulties as the 

 creature becomes highly difterentiated and integrated. Thus it does 

 not occur in Arthropods or Molluscs, nor in any Vertebrates proper, 

 i.e. above the level of the degenerate Tunicates. The outstanding 

 fact implied in having specialised reproductive units is that they 

 have not contributed to the building up of the parental "body", 

 and thus have retained an organisation continuous in quality with 

 the original germ-cell from which the parent arose. They are thus 

 not very liable to be affected by the mishaps which so frequently 

 befall the "body" which bears them. A separated off fraction or bud 

 of Hydra, sea-anemone or planarian has been shown to start with 



Fig. 57. 



The Ikll .\nimalcule, Vorticella. a microscopic Infusorian, often seen as a 

 fringe on waterweed. A, an ordinary individual, .showing the stalk with 

 a contractile fibril (CF) inside a non-contractile sheath (SH). The nucleus 

 (X) is horseshoe-shaped. There are cilia around the mouth; CV, a contrac- 

 tile vacuole; FV, food vacuoles. This individual (II) may produce a bud 

 at the lower end of the bell, and this may divide into eight minute zooids 

 (Z). In III a minute zooid, a microzooid (.M), is seen entering the ordinary 

 individual. This is comparable, mutatis mutandis, to a sperm -cell entering 

 an egg-cell. 



disabihties that the parent body may have acquired, through un- 

 favourable surroundings or food — an obviously undesirable handicap. 

 In the life-history of the germ-cells, and in the mingling of sperm- 

 cell and egg-cell in fertilisation, there is abundant opportunity for 

 new permutations and combinations of hereditary qualities — for 

 variations, in short. Here is surely the crowning advantage of sexual 

 reproduction, that it favours the emergence of the new. Yet perhaps 

 we may look further ahead and recognise that sexual reproduction, 

 in animals especially, leads to separate sexes, often dimorphic, 

 whence courtship, the dawn of the love of mates, and the evolution 

 of parental and other emotions. It must be noted that we are not 

 here forgetting the physiological problems already recognised, nor 

 falling into the teleological fallacy of trying to account for origins 

 by indicating the purposiveness of the new departures. We are 

 merely pointing out the survival values of sexual, as contrasted 

 with asexual, reproduction. 



