LECTURE IV

THE ARGUMENT FROM EMBRYOLOGY

THE last lecture was devoted to the consideration of the evidence afforded by fossils in regard to the theory of Evolution, and may be summed up as follows:-The evidence afforded by fossils is not so complete as we could wish, but we are able to point out the causes which render it difficult or impossible for continuous series to be preserved ; fossils give no evidence against Evolution, and some remarkable series have already been unearthed, such as those of the Horse and Paludina, which would be unintelligible without Evolution; the evidence is steadily increasing in amount and importance; and the evidence of fossils is a disproof of catastrophism.

We are now concerned with the most recent of biological sciences. Embryology, or the Science of Development, is prominently associated with the names of Von Baer, 1792-1881, and Balfour, 18511882. It is utterly vain in one lecture to give any idea of the extraordinary multitude of facts accumulated within the last quarter of a century, or of the numerous and fascinating theories to which these facts have given origin ; it is here merely as bearing

on the doctrine of Evolution that we have to consider Embryology.

There are two great questions to be considered :--First: Does embryology afford evidence for or against the possibility of the descent of animals from unlike ancestors? Secondly: If it gives evidence in favour of such descent, does it afford us any clue in regard to the actual line of descent in a given case, and will it help us to reconstruct the pedigrees or past histories of animals?

The answer to the first question is found in the extraordinary changes which an animal may undergo in its own person, during development, within the space of a few days or weeks, thus showing the possibility of such descent with modification; for instance, the changes which occur during the metamorphosis of the butterfly, and the change of the water-breathing tadpole into the air-breathing frog. This suggests further that such enormous periods of time as are usually demanded to bring about such changes may not really be necessary. A further reply to the question is found in the fact that groups of animals, the relations of which were previously unknown, have had their true zoological positions determined by the study of the changes undergone during their development.

Thus many animals, when adult, present little or no resemblance to other members of the groups to which they really belong. The Ascidians are a wellknown example. So long as their adult condition alone was known, zoologists were entirely in the dark as to their real affinities, and by most writers

they were grouped with the Brachiopoda and Polyzoa, as a subdivision of Mollusca. As soon as their development was worked out, it was found that they were really members of the Vertebrata, inasmuch as when young they show a curious resemblance

b, brain; d, spinal cord; e, eye; i, intestine; m, mouth; n, notochord; 1, pharynx with gill-slits; s, spiracle

to tadpoles-not merely in form and appearance, but in all essential points of structure; while the mode of formation of their nervous system, skeleton, alimentary canal, and other parts, is exactly that obtaining among other Vertebrates, and entirely unlike that of all Invertebrate groups. (Fig. 8.)

Again, we may take animals such as a prawn, a barnacle, and one of those curious sac-like parasites of the genus Sacculina, which are found not uncommonly adhering to the under surface of the rudimentary tail of crabs. These three animals are, when adult, very unlike one another. The prawn is a freeswimming form. The barnacle is fixed firmly to rock,

Stages in Development of the Prawn (Peneus).

A, Nauplius stage; B, Zoæa stage, in which the larva resembles an dult Copepod; C, Schizopod stage, where it corresponds in structure to the adult Schizopoda; D, Adult Peneus.

usually between tide-marks ; it forms a hard protective shell, has no eyes, no locomotor organs, and is hermaphrodite. Sacculina is altogether unlike the other two animals : it has a soft unjointed body, no trace of limbs, no mouth or alimentary canal, and no sense-organs : it is, in fact, merely a soft-walled bag of eggs, attached to the crab's tail by a number of branching root-like

F

processes penetrating the crab's skin and spreading out in its body, from which they absorb the nutriment on which the parasite lives and grows.

Yet when we turn to the development of these animals, we find that, utterly unlike as the adult

Stages in the Development of the Barnacle (Balanus).

A, Nauplius stage; B, Second stage, in which the first pair of swimming appendages of the Nauplius are converted into antenne and the rudiments of the six pairs of cirri appear; C, Pupa stage in this stage the animal is free-swimming and has six pairs of legs, antennules, two large compound eyes, and imperfectly developed masticatory appendages. The pupa becomes attached by its antennules and develops into D, the adult Barnacle. E, Group of Barnacle shells.

forms are, the young of all three genera hatch in the form known as a Nauplius. This Nauplius larva has a short unsegmented body, three pairs of appendages, used for locomotion, and a single median eye; and although the Nauplii are not identical one with another, yet they agree very closely in all