For Aristotle, as for all anatomists before the days of the microscope, the tissues were not much more than inorganic substances, differing from one another in texture, in hardness, and other physical properties. They possessed indeed properties, such as contractility, which were not inorganic, but as far as their visible structure was concerned there was little to raise them above the inorganic level.

The application of the microscope changed all that, for it revealed in the tissues an organic structure as complex in its grade as the gross and visible structure of the whole organism. Of the four men who first made adequate use of the new aid, Malpighi, Hooke, Leeuenhoek, and Swammerdam, the first-named contributed the most to make current the new conceptions of organic structure. He studied in some detail the development of the chick. He described the minute structure of the lungs (1661), demonstrating for the first time, by his discovery of the capillaries, the connection of the arteries with the veins. In his work, _De viscerum structura_ (1666), he describes the histology of the spleen, the kidney, the liver, and the cortex of the brain, establishing among other things the fact that the liver was really a conglomerate gland, and discovering the Malpighian bodies in the kidney. This work was done on a broad comparative basis. "Since in the higher, more perfect, red-blooded animals, the simplicity of their structure is wont to be involved by many obscurities, it is necessary that we should approach the subject by the observation of the lower, imperfect animals."[19] So he wrote in the _De viscerum structura_, and accordingly he studied the liver first in the snail, then in fishes, reptiles, mammals, and finally man. In the introduction to his _Anatome plantarum_ (1675), in which he laid the foundations of plant histology, he vindicates the comparative method in the following words:--"In the enthusiasm of youth I applied myself to Anatomy, and although I was interested in particular problems, yet I dared to pry into them in the higher animals. But since these matters enveloped in peculiar mystery still lie in obscurity, they require the comparison of simpler conditions, and so the investigation of insects[20] at once attracted me; finally, since this also has its own difficulties I applied my mind to the study of plants, intending after prolonged occupation with this domain, to retrace my steps by way of the vegetable kingdom, and get back to my former studies. But perhaps not even this will be sufficient; since the simpler world of minerals and the elements should have been taken first. In this case, however, the undertaking becomes enormous and far beyond my powers."[21] There is something fine in this life of broad outlines, devoted whole-heartedly to an idea, to a plan of research, which required a lifetime to carry out.

An important histological discovery dating from this time is that of the finer structure of muscle, made by Stensen (or Steno) in 1664. He described the structure of muscle-fibres, resolving them into their const.i.tuent fibrils.

To the microscope we owe not only histology but the comparative anatomy of the lower animals. Throughout the 17th and 18th centuries the discovery of structure in the lower animals went on continuously, as may be read in any history of Zoology.[22] We content ourselves here with mentioning only some representative names.

In the 17th century Leeuenhoek, applying the microscope almost at random, discovered fact after fact, his most famous, discovery being that of the "spermatic animalcules."

Swammerdam studied the metamorphoses of insects and made wonderfully minute dissections of all sorts of animals, snails and insects particularly. He described also the development of the frog. It is curious to see what a grip his conception of metamorphosis had upon him when he h.o.m.ologises the stages of the frog's development with the Egg, the Worm, and the Nymph of insects (_Book of Nature_, p. 104, Eng. trans., 1785). He even speaks of the human embryo as being at a certain stage a Man-Vermicle.

In the 18th century, Reaumur and Bonnet continued the minute study of insects, laying more stress, however, on their habits and physiology than upon their anatomy. Lyonnet made a most laborious investigation of the anatomy of the willow-caterpillar (1762). John Hunter (1728-93) dissected all kinds of animals, from holothurians to whales. His interest was, however, that of the physiologist, and he was not specially interested in problems of form. It is interesting to note a formulation in somewhat confused language of the recapitulation theory. The pa.s.sage occurs in his description of the drawings he made to ill.u.s.trate the development of the chick. It is quoted in full by Owen (J. Hunter, _Observations on certain Parts of the Animal OEconomy_, with Notes by Richard Owen. London, 1837. Preface, p.

xxvi). We give here the last and clearest sentence--"If we were to take a series of animals from the more imperfect to the perfect, we should probably find an imperfect animal corresponding with some stage of the most perfect."

The tendency of the time was not towards morphology, but rather to general natural history and to systematics, the latter under the powerful influence of Linnaeus (1707-1778). The former tendency is well represented by Reaumur (1683-1757) with his observations on insects, the digestion of birds, the regeneration of the crayfish's legs, and a hundred other matters. To this tendency belong also Trembley's famous experiments on Hydra (1744), and Rosel von Rosenhof's _Insektenbel.u.s.tigungen_ (1746-1761).

Bonnet (1720-1793) deserves special mention here, since in his _Traite d'Insectologie_ (1745), and more fully in his _Contemplation de la Nature_ (1764), he gives the most complete expression to the idea of the _ech.e.l.le des etres_.

This idea seems to have taken complete possession of his imagination.

He extends it to the universe. Every world has its own scale of beings, and all the scales when joined together form but one, which then contains all the possible orders of perfection. At the end of the Preface to his _Traite_ _d'Insectologie_ (OEuvres, i., 1779) he gives a long table, headed "Idee d'une ech.e.l.le des etres naturels,"

and rather resembling a ladder, on the rungs of which the following names appear:--

MAN.

Orang-utan.

Ape.

QUADRUPEDS.

Flying squirrel.

Bat.

Ostrich.

BIRDS.

Aquatic birds.

Amphibious birds.

Flying Fish.

FISH.

Creeping fish.

Eels.

Water serpents.

SERPENTS.

Slugs.

Snails.

Sh.e.l.l FISH.

Tube-worms.

Clothes-moths.

INSECTS.

Gall insects.

Taenia.

Polyps.

Sea Nettles.

Sensitive plant.

PLANTS.

Lichens.

Moulds.

Fungi, Agarics.

Truffles.

Corals, and Coralloids.

Lithophytes.

Asbestos.

Talcs, Gypsums.

Selenites, Slates.

STONES.

Figured stones.

Crystals.

SALTS.

Vitriols.

METALS.

HALF-METALS.

SULPHURS.

Bitumens.

EARTHS.

Pure earth.

WATER.

AIR.

FIRE.

More subtile matter.

The nature of the transitional forms which he inserts between his princ.i.p.al cla.s.ses show very clearly his entire lack of morphological insight--the transitions are functional. The positions a.s.signed to clothes-moths and corals are very curious! The whole scheme, so fantastic in its details, was largely influenced by Leibniz's continuity philosophy, and is in no way an improvement on the older and saner Aristotelian scheme.

Robinet, in the fifth volume of his book _De la nature_ (1761-6), foreshadows the somewhat similar views of the German transcendentalists. "All beings," he writes, "have been conceived and formed on one single plan, of which they are the endlessly graduated variations: this prototype is the human form, the metamorphoses of which are to be considered as so many steps towards the most excellent form of being."[23]

The idea of a gradation of beings appears also in Buffon (1707-1788), but here it takes more definitely its true character as a functional gradation.[24] "Since everything in Nature shades into everything else," he says, "it is possible to establish a scale for judging of the degrees of the intrinsic qualities of every animal."[25]

He is quite well aware that the groups of Invertebrates are different in structural plan from the Vertebrates--"The animal kingdom includes various animated beings, whose organisation is very different from our own and from that of the animals whose body is similarly constructed to ours."[26]