I may, however, indicate the line of defence I should take were I to go again into the matter. The objections are based on the structure of existing Liverworts and Phænogams. But I have already referred to the probability—or, indeed, the certainty—that in conformity with the general principle set forth in the note to Chapter I, we must conclude that the early types of Liverworts out of which the Phænogams are supposed to have evolved, as well as the early types of Phænogams in which the stages of evolution were presented, no longer exist. We must infer that forms simpler than any now known, and more intermediate in their traits, were the forms concerned; and if so, it may be held that the incongruities with the hypothesis which are presented by existing forms, do not negative it. The scepticism my critic himself expresses respecting the current interpretation is a partial justification of this view. Moreover, his admission that the theory set forth “fits in well with the phenomena exhibited by phanerogamic shoots,” must, I think, be regarded as weighty evidence. On the Evolution hypothesis we are obliged to suppose that the Monocotyledons and Dicotyledons respectively arose by integration of fronds; and if to the question after what manner the integration took place, there is an hypothesis which renders it comprehensible, and agrees both with the structures of the two kinds of shoots and the structures of the two kinds of seeds, as well as with various of the other phenomena the two types present, it has strong claims for acceptance.
Reconsideration suggests the following remarks.
1. Alternation of generations is a means of furthering multiplication. To be effective each member of either generation must be a self-supporting centre of growth or diffusion or both. Hence if, as in the Liverworts, one of the so-called alternating generations is not independent, but a permanent growth on the other—a parasite—it is a misuse of words to call the arrangement Alternation of generations. (Since this was written I have found that Sir Edward Fry takes the same view. He approvingly quotes Professor Bower, who says that “the alternation of generations is not an accurate statement of facts or a useful analogy.”)
2. The alternating of sexual and non-sexual processes is not fundamentally distinctive; for, as shown by sundry Archegoniates, it is an inconstant trait, and as shown by Klebs’ experiments on Vaucheria, the conditions may be varied so as to determine its occurrence or non-occurrence. Nay, the same individual may reproduce in either way.
3. Still more significant is the fact that in some of the marine Thallophytes, there is a process like that which in a moss or a fern is considered an alternation of generations, whereas in others, as the Brown Wrack (Fucus), each generation is sexual. Thus the presence or absence of this mode of genesis cannot be a cardinal distinction.
4. With these facts before us, it is not only a reasonable supposition but a highly probable supposition, that there have existed plants of the Liverwort type in which the so-called alternation of generations did not take place. If so, nearly all the foregoing objections to my hypothesis fall to the ground.]
CHAPTER IV.
THE MORPHOLOGICAL COMPOSITION OF ANIMALS.
§ 199. What was said in [§ 180], respecting the ultimate structure of organisms, holds more manifestly of animals than of plants. That throughout the vegetal kingdom the cell is the morphological unit, is a proposition admitting of a better defence, than the proposition that the cell is the morphological unit throughout the animal kingdom. The qualifications with which, as we saw, the cell-doctrine must be taken, are qualifications thrust upon us more especially by the facts which zoologists have brought to light. It is among the Protozoa that there occur numerous cases of vital activity displayed by specks of protoplasm; and from the minute anatomy of all creatures above these, are drawn the numerous proofs that non-cellular tissues may arise by direct metamorphosis of mixed colloidal substances.[16]
Our survey of morphological composition throughout the animal kingdom, must therefore begin with those undifferentiated aggregates of physiological units [or constitutional units], out of which are formed what we call, with considerable license, morphological units.
§ 200. In that division of the Protozoa distinguished as Rhizopoda, are presented, under various modifications, these minute portions of living organic matter, so little differentiated, if not positively undifferentiated, that animal individuality can scarcely be claimed for them. Figs. [131, 132, and 133], represent certain nearly-allied types of these—Amœba, Actinophrys, and Lieberkühnia. The viscid jelly or sarcode, comparable in its physical properties to white of egg, out of which one of these creatures is mainly formed, shows us in various ways, the feebleness with which the component physiological units are integrated—shows us this by its very slight cohesion, by the extreme indefiniteness and mutability of its form, and by the absence of a limiting membrane. It is no longer held even by unqualified adherents of the cell-doctrine that the Amœba has an investment. Its outer surface, compared to the film which forms on the surface of paste, does not prevent the taking of solid particles into the mass of the body, and does not, in such kindred forms as Fig. [133], prevent the pseudopodia from coalescing when they meet. Hence it cannot properly have the name of a cell-wall. A considerable portion of the body, however, in Difflugia, Fig. [134], has a denser coating formed of agglutinated foreign particles; so that the protrusion of the pseudopodia is limited to one part of it. And in the solitary Foraminifera, like Gromia, the sarcode is covered over most of its surface by a delicate calcareous shell, pierced with minute holes, through which the slender pseudopodia are thrust. The Gregarina exhibits an advance in integration, and a consequent greater definiteness. Figs. [135 and 136], exemplifying this type, show the complete membrane in which the substance of the creature is contained. Here there has arisen what may be properly called a cell: under its solitary form this animal is truly unicellular. Its embryology has considerable significance. After passing through a certain quiescent, “encysted” state, its interior breaks up into small portions, which, after their exit, assume forms like that of the Amœba; and from this young condition in which they are undifferentiated, they pass into that adult condition in which they have limiting membranes. If this development of the individual Gregarina typifies the mode of evolution of the species, it yields further support to the belief, that fragments of sarcode existed earlier than any of the structures which are called cells. Among aggregates of the first order, there are some much more highly developed. These are the Infusoria, constituting the most numerous of the Protozoa, in species as in individuals. Figs. [137, 138, and 139], are examples. In them we find, along with greater definiteness, a considerable heterogeneity. The sarcode of which the body consists, has an indurated outer layer, bearing cilia and sometimes spines; there is an opening serving as mouth, a permanent œsophagus, and a cavity or cavities, temporarily formed in the interior of the sarcode, to serve as one or more stomachs; and there is a comparatively specific arrangement of these and various minor parts.