In order to understand the meaning of conjugation in connection with reproduction, it is important to understand how the two became in the first instance related. For the solution of this question the fact that many Protozoa have the capacity of temporarily or permanently fusing together without an immediate act of reproduction is of great importance. A good example of such fusion is supplied by Actinophrys. We must suppose in fact that the simple coalescence of two or more individuals gives a sufficient amount of extra vigour to their product, to compensate the race for the loss in number of individuals so caused. This extra vigour probably first exhibited itself especially by increased activity in reproduction, till finally the two processes, viz. that of conjugation and that of reproduction, came to be inseparably connected together.

The reproduction of the forms above the Protozoa, which are known as the Metazoa, takes place by two methods, viz. a sexual and an asexual one. The sexual process, which occurs in every known Metazoon[5], consists essentially, as is shewn in the second chapter of this work, in the fusion of two cells budded off from the parent organism, viz. the female cell or ovum, and the male cell or spermatozoon, and of the subsequent division of the compound cell so produced into a number of parts which build themselves up into an organism resembling one of the parents. The sexual process has obviously at first sight a very close resemblance to the process of conjugation. Since it is a question of fundamental importance to determine how sexual reproduction originated, it becomes necessary to examine how far this apparent resemblance is a real one, and how far sexual reproduction can be derived from reproduction following upon conjugation.

[5] Dicyema, if it is a true Metazoon, would seem to form an exception to this rule.

In spite of the general similarity between the two processes there is an obvious difficulty in comparing them, in that the result of conjugation is usually the breaking up of the individual formed by the fusion of two other individuals into a number of new organisms, while the result of the fusion which takes place in sexual reproduction is the formation of a single new organism. This difference between the two processes, great as it is, is perhaps apparent rather than real. It must be remembered that a single individual Metazoon is equivalent to a number of Protozoa coalesced to form a single organism in a higher state of aggregation. It results from this that the segmentation of the ovum which follows the sexual act may be compared to the breaking up of the product of conjugation into spores, the difference between the two processes consisting in the fact that in the one case the spores separate each to form an independent organism, while in the other they remain united and give rise to a single compound organism.

If the above considerations are well founded it seems permissible to accept the general view according to which sexual reproduction is derived from conjugation. It is necessary to suppose that, in a colony of Protozoa in the course of becoming a Metazoon, the capacity of reproduction by spores became localized in certain definite cells, and although the formation of spores from these cells may have been possible without previous conjugation, yet that conjugation gradually became established as the rule. The differentiation of primitively similar conjugating cells into male and female cells was probably a very early occurrence, since indications of an analogous differentiation, as has already been mentioned, are found in certain existing Protozoa (Monads, Vorticella, etc.). I have attempted to shew in the second chapter that the breaking up of the cell into spores without previous conjugation is perhaps provided against in the extrusion of the so-called 'directive body'.

With the differentiation of special germinal cells, to take the place of the whole individual in the act of conjugation, the possibility of each act of conjugation resulting in the production of only a single organism became introduced. Germinal cells can be indefinitely produced, and the reproductive capacity of a single individual is therefore unlimited; while if two whole individuals conjugated and only produced one from the process, the result would be a diminution instead of an increase in the race[6].

[6] In the vegetable kingdom there are numerous types of Thallophytes, which throw a considerable amount of light on the relation between sexual reproduction and conjugation. Subjoined are a few of the more striking cases. In Pandorina at the time of sexual reproduction the cells which constitute a colony divide each into sixteen, and the products of their division are set free. Pairs of them then conjugate and permanently fuse. After a resting stage the protoplasm is set free from its envelope after division into two or four parts. Each of these then divides into sixteen coherent cells and constitutes a new Pandorina colony. In OEdogonium the fertilization is effected by a spermatozoon fusing with an oosphere (ovum). The fertilized oosphere (oospore) then undergoes segmentation like the ovum of an animal; but the segments, instead of uniting to form a single organism, separate from each other, and each of them gives rise to a fresh individual (swarm-spore) which grows into a perfect OEdogonium. In Coleochæte the impregnation and segmentation take place nearly as in OEdogonium, but the segments remain united together, acquire definite cell walls, and form a single embryo. There is in fact in Coleochæte a true sexual reproduction of the ordinary type. (Vide S. H. Vines "On alternation of generation in the Thallophytes." Journal of Botany, Nov., 1879.)

It must be admitted that, in the present state of our knowledge, the passage from reproduction by spores following conjugation, to true sexual reproduction, can only be traced in a very speculative manner, and that a further advance in our knowledge may prove that the steps which I have attempted to sketch out are far from representing the true origin of sexual differentiation. The peculiar conjugation and fusion of two individuals to form Diplozoon paradoxum may be alluded to in this connection. This fusion merely results in the attainment of sexual maturity by the two conjugating individuals. It does not appear to me probable that this conjugation is in any way connected with the conjugation of the Protozoa, but the reverse must be borne in mind as a possibility.

It is not easy to decide whether the hermaphrodite or the dioecious state is the primitive one, or in other words whether the two conjugating cells, from which I have supposed the sexual products to originate, were derived in the first instance from one or from two colonies of Protozoa. On purely à priori grounds it seems probable that they were originally formed in one colony, and that their derivation from two colonies or individuals was inaugurated when the spermatozoon became motile. There can be no doubt that the dioecious state is a very early one, and that the majority of existing cases of hermaphroditism are secondary.

The above considerations with reference to the male and female cells appear to indicate that they were primitively homodynamous; a conclusion which is on the whole borne out by the history of their development.

Although the modes of reproduction amongst the Metazoa have been divided into the classes sexual and asexual, there is nevertheless one mode of asexual reproduction which ought to be classified with the sexual rather than with the asexual modes. I mean parthenogenesis, which consists essentially in the development of the ovum into a fresh individual without previous coalescence with the male element. This mode of reproduction, which has a very limited range in the animal kingdom, being confined to the Arthropoda and Rotifera, is undoubtedly secondarily derived from sexual reproduction. The conditions of its occurrence are discussed in the second chapter.

It is remarkable that in certain cases the absence of fertilization causes the production of males (Bees, a Saw-fly, Nematus ventricosus, etc.); more usually it results in the production of females only, and there are very often in the Arthropoda a series of successive generations of females all producing ova which develop parthenogenetically into females; eventually however, usually in direct or indirect connection with a change of food or temperature, or other conditions, ova are formed which give rise without fertilization both to males and females.

The true asexual modes of reproduction amongst the Metazoa consist of fission and gemmation. Gemmation is by far the most widely disseminated of the two. Various as are the methods in which it takes place, it seems nevertheless that cells derived from all the germinal layers, and very frequently from all the important organs of the adult, assist in forming the bud. Into the details of the process, which require in many points a fuller elucidation, it is not my purpose to enter.