575.1 305 
I 
The Fundamental Principles of Heredity 
(Concluded from p. 239) 
FT XHE power of propagation of animals by small fragments is pos- 
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 with swim- 
ming lashes. The brood-cells may in one and the same species! 
1 The filamentous Alga Ulothrix zonata. 
af 
