192 THE EVOLUTION OF SEX. 



Asexual reproduction or multiplication by more or less dis- 

 continuous 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. — Fertilisation began in almost mechanical fusion. Reproduc- 

 tion 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 

 Sthizogenes, but it also occurs in a few of the relatively high infusorians. 

 That the breakage sometimes means dissolution is certain ; nor is reproduc- 

 tion 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 over- 

 flow. 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, e.g., 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 behaviour 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, cannot 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 protoplasm. 



Sponges. — In sponges no one can fail to recognise 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 out- 

 grown tube may lose connection with the parent, or a great tumour-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 form gemnmles, which often save the life of the otherwise dying 

 sponge. They are complex enough, with sheaths and spicules, and some- 

 times even with a float, but in principle they simply do by a multiple union 

 what is otherwise attained by ovum and sperm. Best known in this 



