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♦ KNOWLEDGE ♦ 



[September 1, 1886. 



those of birds ; in the infinite division of organs, the spider 

 spinning the six hundred strands of its web from as many 

 teats ; the dragon-dy with its twelve thousand eyes, each 

 with its own lens and cone and rod ; the caterpillar, with 

 its fifteen hundred air-tubes ; we learn that magnitude is 

 not necessary to complexity. In the high nervous organisa- 

 tion of insects and the variety of functions, many of these 

 quasi-human, which they discharge; in the dexterity of 

 their actions, and the manifest adaptation of means to ends ; 

 in the social order of certain species, notably the ant-com- 

 monwealth with its division of labour, its slave and fighting 

 population, its nurseries for pets and weaklings — a political 

 and industrial order, which has not, like ours, to readjust 

 itself by peaceful or bloody revolutions to changing con- 

 ditions— we have striking evidence of the inter-relation of 

 all things living, and of the imreality of the distinctions 

 which man has set up between instinct and reason ; in fine, 

 evidence of fundamental correspondence between the nervous 

 sy.stem3 of the lowest and the highest. Complexity, not size ; 

 mental, not physical power, mark advance in the organism ; 

 and it is in the' specialisation of the nervous system, and in 

 the proportion of its controlling centre, the brain, to the rest 

 of the structure, that the mechanical explanation of intelli- 

 gence lies.* Mr. Darwin remarks that the brain of an ant, 

 which is proportionally larger than that of any other insect, 

 although itself not so large as the quarter of a small pin's 

 head, " is one of the most marvellous atoms of matter in the 

 world, perhaps more so than the brain of a man." 



There is much force in the argument that the long period 

 of infancy, with its consequent dependence on parental love 

 and care, through which man, and in lesser degree the highest 

 ajies and other animals pass, has tended to develop the feeling 

 of sympathy and its expression Ln service of the helpless by 

 which the f imily is knit together, and out of which has grown 

 the social instinct which forms tribes and nations. And 

 the argument does not stop here. The more intelligent the 

 animal the longer is the baby stage, for where there is a 

 complex nervous system its specialisation goes on after birth ; 

 whereas in the case of an animal with low capacities all the 

 nervous connexions are formed before biith, so that it begins 

 life in lusty independence, fully equipped for work, and 

 therefore wich no tie to bind it to its parents, while its iso- 

 lated life is fiital to mental development. 



Now the ant, with other communal insects, as bees and 

 wasps, has to pass through a relatively long grubhood, and 

 in this we may have the explanation of its high social organi- 

 sation, which has had measureless time for its development, 

 since the remains of the hymenoptera are found as far back 

 as the Jurassic age, i.e. the middle of the Secondary Epoch. 

 And if the argument has any force in the case of man, the 

 evolutionist is bound to apply it to the ant. 



Hut in the highest members of the annulosa we 

 arrive at the extremity of one branch of the life-tree, and 

 we must descend to reach the starting-point which leads us 

 to the loftier branch whose topmost twig is man. 



Y. MoLi.uscA. — This sub-kingdom, the importance of 

 whose fossil remains has been indicated, includes a wide 

 lange of organisms, any common definition of which is 

 diflicult. Many of them appear, like the fallen angels, not 

 to have kept their first estate, as, e.;/., the lowest class, which 

 resembles polyps and was formerly erroneously grouped with 

 them. In the larger number of molluscs symmetry of form 

 is more the exception than the rule, and, in one class, to 

 be dealt with presently, we have the nearest known ally 

 of the vertebrates. 



Some of the moUusca have neither heads nor hearts, or at 



* The proportionate weight of brain to body is, in Fishes, 1 to 

 5,600; Reptiles, 1 to 1,300; Birds, 1 to 200; Mammals, 1 to 180. 



least quite imperfect ones, others have heads and chambered 

 hearts ; some grow together in colonies, others live an 

 independent life ; but all are alike soft-bodied, lacking the 

 jointed structure which distinguishes the annulosa. Some, 

 as the sea and land slugs, are naked, although furnished with 

 a delicate shell when young ; others have a leathery or 

 gi'istly covering; the rest, the shell-fish proper, are pro- 

 tected by single or double valves, which in their spiral forms 

 and fadeless colouring sometimes surpass the loveliest flowers, 

 or which, as in the pearl-oyster, yield the lustrous substance 

 which, according to ancient fable, is formed of raindrops 

 falling into the open valve, where some mysterious agency 

 transmuted them. The power of secreting matter from the 

 surrounding water for the construction of their shells is ono 

 of the most persistent characteristics of living things, and in 

 all the moUusca the shells (which are not cast periodically, 

 as with the Crustacea) are secreted along the surface of the 

 thick flexible skin called the " mantle," the crumpled form 

 of which determines their shape. They range in size from 

 the enormous Tridacna of tropical seas, which sometimes 

 weighs five hundred pounds, to the minute species of our 

 coasts, thousands of which would scarcely exceed an ounce 

 in weight. 



The lowest molluscs are the plant-like, fixed Sea-mats and 

 ,^ea-jnosses ; the highest are represented by the Briarean 

 Cuttlefish, horn the common species of our seas to the octopus 

 with its rudimentary internal skeleton and its chameleon- 

 like power to change its colour ; and by the pearly Nautilus, 

 the survivor of an ancient family that swarmed in the 

 waters of the Jurassic and Cretacean periods. Between these 

 range the more familiar shell-fish, notably the oyster, which, 

 in common with all bivalves, is headless ; and the periwinkle, 

 whose land congener is the air-breathing snail. 



But, for the evolutionist, interest in this sub-kingdom 

 centres in the transparent bag-shaped Sea-squirts, or ascid- 

 ians, which, although classed under Tunicata (Lat. tunica, a 

 cloak), are entitled to a distinctive place. Most of the sijecies 

 are immobile, attaching them.selves to rocks, shells and other 

 objects, sometimes growing .separately, sometimes in clusters 

 on a common stem. Of the two openings in their gristly 

 covering, which, by the way, is largely made up of cellulose, a 

 characteristic element in plants, one is the mouth and the 

 other the vent. The mouth opens into a breathing sac, fur- 

 nished with numerous gill-slits and cilia, and leading through 

 the gullet to the digestive organs— stomach and intestine — 

 which are connected by a sharp bend with the vent, whence 

 the inhaled water, after giving up its oxygen to the blood, is 

 expelled. The heart, a tube-shaped organ, is placed at the 

 lower end of the body-cavity which fills the space around 

 the intestine, and the nervous system, consisting of a single 

 ganglion, lies between the mouth and vent. The position 

 of this ganglion, as will be seen later on, gives an 

 important clue to the connexion between the ascidians 

 and the vertebrates, but still more important evidence as 

 to this is supplied in the early stages of the ascidiau's 

 development. In certain species the egg gives rise to a 

 larva resembling the tadpole of a frog, both outwardly and 

 inwardly, a resemblance " reaching absolute identity when 

 we examine the way in which the various organs arise from 

 the primitive egg-cell," the only important diflerence being 

 that the ascidian has but one eye. The larva of the ascidian 

 and the frog alike possess four structures which are common 

 to every backboned animal at some stage of its development, 

 and the possession of which is explicable only on the theory 

 of the descent of sea-squirts and vertebrates from a common 

 ancestor. These four structures are (1) the throat with 

 its gill-slits; (2) the primitive backbone— a gristly rod 

 called the notochord which is found in no invertebrates 

 except the ascidians ; (o) the brain and spinal cord ; and 



