659 



REPRODUCTION 



iu continuity ; the races of animals and the orders ; 

 of plant* live mi without nmiked change for j 

 centuries; hy slow modifications they may be! 

 enriched or iiii|x>verislu*<i, increased or thinned, but 

 there is no breach of continuity. All the forms of 

 life seem to evolutionists like twigs on one many- 

 hranched tree ; they are genetically related by near 

 or distant bonds of kinship, and in a very real 

 seniie each generation is continuous with those 

 which come before and after it. As an evergreen 

 tree replaces by fresh growth those leaves which it 

 loses, so, throughout the world, nv various forms of 

 reproduction the continuance of life is secured. 



As reproduction is a fundamental fact of life, it 

 cannot be discussed apart from other fads, such as 

 growth, at the limit of which reproduction usually 

 occurs, or development, in which the germ grows , 

 into the likeness of its parent, or the occurrence of j 

 two sexes producing complementary and mutually 

 dependent reproductive elements. A theory of 

 reproduction must be consistent with the facts of 

 growth and development, and merges into theories 

 of sex and of heredity the latter being based on a 

 study of the precise relations between successive 

 generations. See EMBRYOLOGY, HEKKIUTY, SEX. 



Modes of Reproduction. Separated fragments of 

 a sixmge or cuttings from the rose, the buds of a 

 hydra, or the bulbils of a lily, the eggs of birds, and 

 the seeds of plants are alike aide to grow into new 

 organisms ; and thus we see that the common fact 

 aliout all kinds of reproduction is that parts of one 

 organism are se|>arated to form or to help to form 

 new lives. In many cases what is separated from 

 the parent life is simply part of ii- body, an over- 

 growth or a definite hud, which, being set free, is 

 able to reproduce the whole of winch it is a re- 

 presentative sample. This we call asexual repro- 

 duction. In most cases, however, the parents give 

 origin to special reproductive elements egg-cells 

 and male-cells which combine and are together 

 able to grow into a new life. This we call sexual 

 reproduction. 



The simplest forms of reproduction are found 

 among the single-celled plants and animals. There 

 we may find an organism like Schizogenes, multiply- 

 ing by breakage, reproducing by rupture, presum- 

 ably when the cell has overgrown its normal size ; 

 iu others numerous hud- are liberated at once, as 

 in Arcella and Pelomyxa ; in many, familial K in 

 the yeast-plant, one bud is formed at a time ; in 

 most the cell divides into two or many daughter- 

 cells. The formation of many daughter-cells or 

 spores is little more than ordinary division taking 

 place repeatedly in rapid succession, and within the 

 substance of the parent-cell in other words, in 

 liiiiil'-'l tun.- ami sjiivce. 



We have seen that reproduction begins among 

 single-relied organisms in a kind of rupture ; but 

 'vii among the more complex forms of life an 

 equally crude mode of reproduction sometimes 

 "in:-. The cast-off arm of a starfish may re-grow 

 the entire animal with a readiness that suggests a 

 habit ; some kinds of worms (e.g. Nemerteans ) 

 break into pieces, each of which is able to re-grow 

 the whole ; large pieces of a sea-anemone or of a 

 ponge are sometimes separated oil' and form new 

 nigaiiisiiiH. It is easy to show c\|>eiimcntally that 

 parts cut from a hydra, a s|M>nge, or a sea-anemone, 

 from a seaweed, a moss, or a tree, may in certain 

 conditions grow into an entire organism. 



Hut the usual mode of asexual reproduction is by 

 the formation of definite buds. A\ hen these buds 

 remain continuous, colonial organisms result, like 

 many s|x>nges, most hydroids, Siphonophora like 

 the Portuguese man-of-war, many corals, almost 

 all the Polyzoa, and many Tnnicates. The runners 

 of a Htrawlirrry and the suckers which grow around 

 a roue bush illustrate the same state. But in a 



few plants, like the liverwort and the tiger-lily, a 

 kind of bud may be detached, and thus begin a 

 new life. It is among animals, however, that the 

 liberation of buds is best illustrated, for this mode 

 of leproduction occur* in hydra and many hydroids, 

 in some ' worms,' and in Polyzoa, and even in 

 animals as highly organised as Tnnicates. Budding 

 is usually exhibited by comparatively simple and 

 by sedentary animals, and seems indeed to be 

 natural to vegetative organisms. It is easy to 

 understand why asexual reproduction is among 

 many-celled animals always associated with sexual 

 reproduction, and entirely replaced by it in the 

 higher forms. For budding is only possible when 

 the organism is not very highly differentiated, or 

 when part of the body retains many indillerent 

 units ; moreover, it is an expensive way of securing 

 the continuance of generation, and is without the 

 advantage to the species which undoubtedly results 

 from the mingling of two life cHiients in sexual 

 reproduction. 



Sexual reproduction in its fully differentiated 

 form involves (a) the distinctness of two parent 

 organisms, i //) the formation of two different kinds 

 of reproductive elements e.g. spermatozoa pro- 

 duced by the male and ova by the female, anu(c) 

 the fertilisation of the egg-cell by a male element. 

 Moreover, the process of sexual reproduction also 

 includes the sexual union of the two parents, or 

 other ways in which fertilisation is secured, while 

 in some coses the fertilised ovum develops in 

 organic relation with the mother-organism, from 

 which it is eventually separated as an embryo. 

 But, while many organisms exhibit fully differen- 

 tiated sexual reproduction, and while the essentials 

 of the process are always the same, there are not a 

 few important variations in detail witness the 

 occurrence of hernmphroditism, parthenogenesis, 

 and alternation of generations, the first and last of 

 which are discussed in separate articles. 



Physiology of Reproduction. All growth is, in a 

 certain sense, of the nature of repriwuction. It is 

 an increase in the amount of protoplasm and its 

 attendant train of substances. Abundance of food 

 material and conditions favourable to rapid assimi- 

 lation are necessarily accompanied by rapidity of 

 growth ; but in the most favouring circumstances 

 there is an inevitable limit to the growth in size of 

 a single cell. It occurs when the rate of assimila- 

 tion of the constantly increasing moss of protoplasm 

 becomes equal to the highest possible rate of absorp- 

 tion. Since absorption can only take place through 

 the surfaces, and since, with any given figure of cell, 

 the ratio of volume to surface is a perfectly definite 

 one, and one which increases at a definite rate as 

 the cell grows, there must be for any fjiven figure 

 of cell a perfectly definite limit of size. For any 

 mass of cells arranged in any manner there must 

 be, for similar reasons (though other factors, such as 

 weight, &c., nitty be operative and varyingly im- 

 portant), a definite limit of si/e. \Vhen in the 

 single-celled animals this limit is reached, or is nearly 

 reached, so that starvation begins and in any: ea-e 

 the greater the size of the cell the less rapid, in 

 proportion to volume, must be the absorption, 

 unless at a certain point other factors at present un- 

 known occur then division of the cell takes place, 

 by which means, the volume remaining the same, 

 the surface is doubled, so that the ratio of volume 

 to surface and therefore of assimilation to absoip- 

 tion is lowered, and growth is once more ptmibM, 

 This law (first clearly stated by Spencer and by 

 I.euckart) is evidently the expression of a factor 

 concerned in the initiation of cell division and 

 therefore of the Metazoa or many-celled animals. 

 In the Protozoa, then, reproduction is related to, 

 and in a certain sense caused by, a diminution ill 

 the possible rate of assimilation, which, to the 



