of the lower animals feed so easily that economy is not of any account. A single Hydroid polyp developed from a fertilised ovum soon builds up asexually a great colony of thousands of individuals. It must also be remembered that the economy of sexual as against asexual reproduction is in many cases only apparent — the wastage of germ-cells being frequently enormous. (b) A second suggestion is that asexual reproduction is incompatible with the complex organisation of a higher animal. We cannot think of a bird multiplying by fission or of an elephant giving off a bud! They are too highly differentiated. No piece of a higher animal could serve as a representative sample of the whole, as an excised piece of polyp does. The possibility of keeping pieces of tissue alive after excision from the body is very much greater than was formerly supposed, but a separated piece of a higher animal is not in natural conditions viable. Now there is doubtless good sense in this suggestion, but it cannot be the complete answer. This must be granted when it is noted that some very complex organisms give off buds, witness the bulbils of the tigerlily, a highly evolved flowering plant, or the remarkable buds of the beautiful free-swimming Tunicates known as Salps. The Salp is a very intricate animal, with a much specialised body, and yet it alternates reproduction by germ-cells with reproduction by budding. This is effected by means of a reproductive "stolon" - a shoot-like outgrowth containing prolongations from most of the important organs in the body and becoming

eventually separated into a chain of buds. In other words, a means has been evolved whereby an animal with a complex body may continue to produce buds, just as simple polyps do. (c) The third suggestion is that the distinction between germ-cells and body cells would justify itself in leaving a free course for the differentiation of body cells (as nervous, contractile, glandular, and so on), and in securing that the reproductive cells were in some measure at least sheltered from the influences of the accidents and incidents of

bodily life. Reproduction by special germcells obviates or lessens the risk, attendant on asexual reproduction, that the offspring share in the acquired bodily defects of the parent. Of course it must likewise operate against the transmission of extrinsically acquired bodily gains. On the other hand, the important constitutional gains which begin as germinal variations are more or less securely entailed by the germ-cell method, in virtue of what is called the continuity of the germ-plasm (see our Evolution, p. 114). Variations are indeed common among asexually produced plants, as we see at a Chrysanthemum show, but the germ-cell method is even more productive. Some recent work suggests that the nutritive and other environment of the young germ-cells within the body may serve to provoke germinal variations, acting in fact as "liberating stimuli." Thus the risk of transmitting bodily defects is lessened, while the supply of heritable changes of endogenous origin is increased. Another advantage probably lies in the fact that the

commencement of the new individual life at the level of a single cell means an ever recurrent re-unification of the inheritance. For no one can think of the germ-cell as like an ill-packed portmanteau with a higgledy-piggledy of hereditary items: it is a living creature in a condensed form, an individuality with a unified organisation. So far, then, an answer to the question: In what respects is sexual reproduction an improvement on asexual reproduction?

SPERM-CELLS

AND EGG-CELLS. The next step was also a confirmation of what some form like Volvox began the definite establishment of dimorphic germ-cells or gametes. One type of germ-cell was relatively large, with a good deal of cytoplasm (or general cell-substance) in proportion to nucleus, provided with some store of nutritive material, and therefore sluggish - the Ovum. The other type of germ-cell was relatively small, with little cytoplasm in proportion to nucleus, with very little reserve material, and therefore predisposed to move towards nutritive substances, to explore the Spermatozoon. This evolution of dimorphic gametes, with antithetic but complementary qualities, was an echo of the macro- and micro-spores of some Protozoa, but there was, without doubt, a fresh start made when animals with a "body" came upon the scene.

In most animals the infinitesimally minute and intensely active spermatozoa seek out the ova which, though often invisible to the unaided eye, are relatively large. The spermatozoon is often only 100000 of the size of

the ovum. In the flowerless plants, such as ferns and mosses, there are actively locomotor spermatozoa or antherozooids (see Fig. 6), but though there long seemed to be no inter

FIG. 5.-Ovum and spermatozoon (S) of a sea-urchin, alike magnified over 750 times, showing relative size. (After E. B. Wilson.) The ovum shows the cell-substance or cytoplasm (C) with an intricate structure, the nucleus (N) with readily stainable bodies known as chromatin threads (CHR), and with a nucleolus or germinal spot (GS).

mediate step between this and the seeming static pollen grain of higher plants, a mobile spermatozoon has been found to emerge from the pollen tube in certain cycads, and also in the strange Gingko or Maiden-hair Tree brought to our gardens from the temple en

closures of old Japan. And the pollen-grains of ordinary flowering plants, although they correspond to the whole male generation, and usually abbreviate away the formation of spermatozoa in adaptation to the loss of watery

FIG. 6.-Female reproductive organ or archegonium in the Adder's Tongue Fern-Ophioglossum -and the motile male cells or antherozooids (SP). The egg-cell is marked OV; it lies in a flask-like recess, the neck of which shows a mucous secretion through which the male cells make their way. (After Bruchmann.)

environment, are adapted to passive transportation, from blossom to blossom, by the wind or by the agency of insects (Fig. 7).

To the question why the dimorphism of germ-cells should have been justified in the course of evolution, two answers may be given. (a) A number of facts point to the