190 THE EVOLUTION OF SEX. 



molester of a sea-cucumber by the ejection of its viscera ; or a 

 tetanic contraction of the muscles makes the slow-worm brittle 

 in the hands of its captor. The power of regeneration is most 

 marked in echinoderms, but persists as high up as reptiles. 

 The regrowth of part of a lizard's leg is the chef-d'csuvrc in this 

 line. Beyond that, regeneration is restricted to little things. 

 We constantly regenerate the skin of our lips, but we cannot 

 naturally replace an amputated limb. It is more marvellous 

 that we cannot, than that the lizard can. That the cells of an 

 irritated stump should divide and multiply, and that the result 

 should be the same as it was at the first, is really no marvel, or 

 rather as much as, but no more than the original development. 

 The dividing cells of the growing stump are simply repeating 

 their original development. 



§ 3. Degrees of Asexual Reprod2iction. — The keynote of the 

 subject was truly struck by Spencer and Hseckel, when they 

 defined asexual reproduction as discontinuous growth. All 

 growth is a reproduction of the protoplasm and its nuclear 

 elements, or in short of the cells ; all reproduction (excluding 

 the important fact of fertilisation) is growth. The ovum, 

 asexually produced from the parent ovum or its lineal de- 

 scendant cells, grows and reproduces itself in turn, building up 

 the embryo. The embryo grows into an adult organism, and 

 the surplus of continued growing energy results in the asexual 

 production of buds, or the sexual discharge of differentiated 

 reproductive elements. We start from the ordinary processes 

 of cell-multiplication and regeneration exhibited in the normal 

 organism. Then come the processes by which lost members 

 are regenerated, involving more or less serious extra growth. 

 To these we must add the rarer and yet not rare cases, where 

 the artificial halves or fractions of an organism can grow into 

 wholes. Normal and frequent however are the very abundant 

 cases of budding, where a sponge or hydra, zoophyte or coral, 

 has surplus enough to grow off new individuals, which remain 

 continuous with itself. The parent organism, whether zoophyte 

 or strawberry-plant, has an asexually produced progeny round 

 about, and in asexual continuity with itself. But they do not 

 always remain continuous ; the hydra produces buds, but 

 eventually sets them adrift. This is still better seen in many 

 of the hydroids, where individuals are separated off as swim- 

 ming-bells or medusoids. The multii)lication has become 

 discontinuous. Continue the process, and we find the libera- 



