l 7 8 THE EVOLUTION OF SEX. 



reproduce by bulbils, arising upon the frond; and the bulbils which 

 arise in the axils of the leaves of the tiger-lily are familiar missiles for 

 every child accustomed to a flower-garden (see figs. pp. 209 and 264). 

 The alliums, and some of our common grasses also, furnish us with 

 examples of the replacement of flowers by separable buds. Asexual 

 reproduction or multiplication by more or less discontinuous growth, 

 without the differentiation of special and mutually dependent sex-cells, 

 occurs from the simplest animals on to the tunicates or sea-squirts, 

 from the base to just over the line which separates backboneless and 

 backboned animals. It is necessary, however, to review the groups. 



Protozoa. — Fertilization began in almost mechanical fusion. Reproduction 

 begins with almost mechanical rupture. The unit-mass of protoplasm, becoming 

 too big for control, breaks. Thus it saves itself and at the same time multiplies. 

 Such breakage may be seen in a primitive form like Schizogenes, but it also 

 occurs in a few of the relatively high infusorians. That the breakage some- 

 times means dissolution is certain ; nor is reproduction ever so very far removed 

 from death. 



The rupture becomes orderly and systematic in budding. This may be 

 multiple, as in the common Arcella, where a number of small buds are 

 constricted off all round. But the process is oftener concentrated in one 

 extrusion or overflow. In budding, the separated daughter-cell is in varying 

 degree smaller than the parent, and the process resembles an overflow. When 

 the bud is approximately equal to the parent, and the process is of the nature of 

 a constriction, it is of course division. 



The division may also be multiple, taking place in rapid succession and in 

 limited space, for example, within a cyst. Then we speak of spore-formation. 

 The last three modes of multiplication are exceedingly common among 

 Protozoa. 



These buddings and divisions are not, of course, rough and ready processes. 

 The nucleus almost always shares in them in an orderly and deliberative 

 fashion. There are variations in its behavior as in higher animals, but there is 

 no doubt that cell-division, with a gradient of progress like everything else, is 

 essentially one and the same in the vast majority of cases. Gruber has been 

 especially successful in proving that fragments of Protozoa, artificially separated 

 without nuclear elements, can not live long, though they may grow and repair 

 their losses for a little. The nucleus is essential to life, though sometimes it 

 seems to disappear, and become as it were a diffuse precipitate in the proto- 

 plasm. 



Sponges. — In sponges no one can fail to recognize the impossibility of 

 drawing any rigid line between growth and asexual reproduction. Between 

 simple extension of the parent mass, and the budding off of new individuals, 

 no sure distinction can in many cases be made out. Sponges do not divide, 

 though they may be cut up, yet they give off discontinuous buds. An outgrown 

 tube may lose connection with the parent, or a great tumor-like mass may be 

 slowly extruded, or tiny brood-buds may be set adrift to shift for themselves. 

 In disadvantageous conditions the surface of a sponge sometimes gathers into 

 minute superficial buds, by means of which it is possible that the life is saved. 



In the fresh-water sponges, in disadvantageous circumstances, — of cold in 

 some countries, heat and drought in others, — some of the cells club together to 



