Adam Sedgwick

“ Just as the adult may be supposed, on evolution doctrine, to be derived from an ancestral adult, so the various larval stages may be supposed to have been derived from the corresponding larval stage of the hypothetical ancestor. If we admit organic evolution at all, we may perhaps go so far, but we are not in a position to go further, and to assert that each larval stage is representative of and, so to speak, derived from some adult stage in the remote past, when the organism progressed no further in its life-cycle than the stage of structure revealed by such a larval form. ”

“ It may further be asserted that land and fresh-water animals develop without a larval stage much more frequently than marine forms. This is probably partly due to the fact that the conditions of land and fresh-water life are not so favourable for the spread of a species over a wide area by means of simply-organized larvae as are those of marine life, and partly to the fact that, in the case of fresh-water forms at any rate, a feebly-swimming larva would be in danger of being swept out to sea by currents. ”

“ Again in certain Trematoda larval stages known as the sporocyst and redia produce ova which have the power of developing unfertilized; in this case the larva probably has not the power of continuing its development. It is very generally held by philosophers that the end of life is reproduction, and there is much to be said for this view; but, granting its truth, it is difficult to see why the capacity for reproduction should so generally be confined to the later stages of life. We know by more than one instance that it is possible for the larva to reproduce by sexual generation ”

“ Has the duration and complexity of the life-cycle expanded or contracted since organisms first appeared on the earth? According to the current view, the life-cycle is continually being shortened at one end by the abbreviation of embryonic development and by the absorption of larval stages into the embryonic period, and lengthened at the other by the evolutionary creation of new adult phases. ”

“ The essential condition of success in this process is that the organism should always shift into the environment to which its new structure is suited, any failure in this leading to impairment of the organism. ”

“ In the majority of cases, however, in which there is a small amount of food yolk and no special arrangements for parental care, a larva is formed. No better group than the Mollusca can be taken to illustrate this point, for in them we find every kind of development from the completely embryonic development of the Cephalopoda, with their large heavily-yolked eggs, to the development of most marine Lamellibranchiata and many Gastropoda, in which the embryonic period is short and there is a long larval development. ”

“ During the embryonic phase the development occurs under protection, either within the egg envelopes, or within the maternal body, or in a brood pouch. At the end of this phase the young organism becomes free and uses, as a rule, its own mouth and digestive organs. ”

“ We may perhaps have a right to take up this position, but it is of no advantage to us to do so, because it leads us into the realm of pure fancy. Moreover, it assumes that an answer can be given to the question asked above—has the life-cycle of organisms contracted or expanded as the result of evolution? ”

“ The two great groups, Annelida and Mollusca, which by their adult structure present considerable affinity with one another, agree in possessing a very similar larval form, known as the trochosphere or trochophore.
A typical trochosphere larva (figs. 1, 2) possesses a small, transparent body divided into a large preoral lobe and a small postoral region. The mouth (4) is on the ventral surface at the junction of the preoral lobe with the hinder part of the body, and there is an anus (7) at the hind end. ”

“ In most cases the shifting of the environment is a very gradual process, and the morphological changes in connexion with each step of it are but slight. In some cases, however, jumps are made, and whenever such jumps occur we get the morphological phenomenon termed metamorphosis. ”

“ From the above considerations it is not surprising to find that the larvae of different members of any group resemble each other to the same kind of degree as do the adults, and that the larvae of allied groups resemble one another more closely than do the larvae of remote groups, and finally that a study of larvae does in some cases reveal affinities which would not have been evident from a study of adults alone. ”

“ This is only what would be expected when it is remembered that the developmental changes never cease. It might be held that the presence of functional reproductive organs, or the possibility of rapidly acquiring them, marks off the adult phase of life from the larval. ”

“ In animals with a well-marked larval phase, by far the greater number of the stages of structure are included in the larval period, for the developmental changes are more numerous and take place with greater rapidity at the beginning of life than in its later periods. ”

“ These terms, if they have any value, are applicable with equal force to adults, but they are cumbrous, and the absence of any satisfactory test which enables us to distinguish between a character which is ancestral and one which has been recently acquired renders their utility very doubtful. ”