575.1 305 



The Fundamental Principles of Heredity 



( Concl uded from p. 239) 



rr^HE power of propagation of animals by small fragments is pos- 

 -L sessed very largely by Sponges, some Coelenterates, Starfishes 

 and certain Flat worms ; it is practically lost in the higher groups for 

 several reasons, considerations of nutrition being most important. 

 An Animal fragment can only obtain the nutritive matter for form- 

 ing new cells by eating up, as it were, part of itself, until it has 

 formed new organs for the prehension and digestion of food. To do 

 this, the fragment must be always big enough to render this sacri- 

 fice possible ; and, moreover, the tissue-cells must not be too special- 

 ised to adapt themselves to the altered conditions. Thus, the 

 complex tissues of a human arm, accustomed to be served by a con- 

 stant supply of blood current bearing in an abundance of food and 

 oxygen and carrying off all waste materials, and to the guidance of 

 a highly developed nervous system, can never adapt themselves to a 

 life of isolation. In this respect Animals contrast markedly with 

 Plants. 



To study in the way we have applied to Animals the laws of 

 reproduction and propagation in Plants, we must revert to those 

 Protists whose life is essentially vegetal. These possess a coloured 

 portion of protoplasm (green, yellow, or red), in which, under the 

 stimulus of light, inorganic materials are combined to form the 

 organic food on which (like animals) they feed. As these inorganic 

 materials exist in solution, they can soak into the cell, which needs 

 neither mouth nor stomach ; and the cell can exist, grow, and 

 multiply by division at the limit of growth, even while invested 

 with a thin coating of the papery material, cellulose. If the cell 

 start as a cylinder or ovoid, and the divisions are always in the 

 same direction, at right angles to its length, the product (a colony of 

 our first type) is an elongated filament, like those which form the 

 green, slimy scum on our way-side ditches ; if the divisions take place 

 in two planes, the colony will form a plate or disk ; if in three, a solid 

 mass which is much more rare. When a period of increased vital 

 activity ensues, brood-formation sets in ; the brood- cells are at first 

 naked, lacking the cellulose wall, and usually provided wiih swim- 

 ming lashes. The brood-cells may in one and the same species 1 



1 The filamentous Alga Ulothrix zonata. 

 Y 



