The Evolution of Man, vol 2 | Page 2

VERTEBRAL COLUMN.
FIGURE 2.328. PIECE OF THE DORSAL CORD.
FIGURES 2.329 AND 2.330. DORSAL VERTEBRAE.
FIGURE 2.331. INTERVERTEBRAL DISK.
FIGURE 2.332. HUMAN SKULL.
FIGURE 2.333. SKULL OF NEW-BORN CHILD.
FIGURE 2.334. HEAD-SKELETON OF A PRIMITIVE FISH.
FIGURE 2.335. SKULLS OF NINE PRIMATES.
FIGURES 2.336 TO 2.338. EVOLUTION OF THE FIN.
FIGURE 2.339. SKELETON OF THE FORE-LEG OF AN AMPHIBIAN.
FIGURE 2.340. SKELETON OF GORILLA'S HAND.
FIGURE 2.341. SKELETON OF HUMAN HAND.
FIGURE 2.342. SKELETON OF HAND OF SIX MAMMALS.
FIGURES 2.343 TO 2.345. ARM AND HAND OF THREE ANTHROPOIDS.
FIGURE 2.346. SECTION OF FISH'S TAIL.
FIGURE 2.347. HUMAN SKELETON.
FIGURE 2.348. SKELETON OF THE GIANT GORILLA.
FIGURE 2.349. THE HUMAN STOMACH.
FIGURE 2.350. SECTION OF THE HEAD OF A RABBIT-EMBRYO.
FIGURE 2.351. SHARK'S TEETH.
FIGURE 2.352. GUT OF A HUMAN EMBRYO.
FIGURES 2.353 AND 2.354. GUT OF A DOG EMBRYO.
FIGURES 2.355 AND 2.356. SECTIONS OF HEAD OF LAMPREY.
FIGURE 2.357. VISCERA OF A HUMAN EMBRYO.
FIGURE 2.358. RED BLOOD-CELLS.
FIGURE 2.359. VASCULAR TISSUE.
FIGURE 2.360. SECTION OF TRUNK OF A CHICK-EMBRYO.
FIGURE 2.361. MEROCYTES.
FIGURE 2.362. VASCULAR SYSTEM OF AN ANNELID.
FIGURE 2.363. HEAD OF A FISH-EMBRYO.
FIGURES 2.364 TO 2.370. THE FIVE ARTERIAL ARCHES.
FIGURES 2.371 AND 2.372. HEART OF A RABBIT-EMBRYO.
FIGURES 2.373 AND 2.374. HEART OF A DOG-EMBRYO.
FIGURES 2.375 TO 2.377. HEART OF A HUMAN EMBRYO.
FIGURE 2.378. HEART OF ADULT MAN.
FIGURE 2.379. SECTION OF HEAD OF A CHICK-EMBRYO.
FIGURE 2.380. SECTION OF A HUMAN EMBRYO.
FIGURES 2.381 AND 2.382. SECTIONS OF A CHICK-EMBRYO.
FIGURE 2.383. EMBRYOS OF SAGITTA.
FIGURE 2.384. KIDNEYS OF BDELLOSTOMA.
FIGURE 2.385. SECTION OF EMBRYONIC SHIELD.
FIGURES 2.386 AND 2.387. PRIMITIVE KIDNEYS.
FIGURE 2.388. PIG-EMBRYO.
FIGURE 2.389. HUMAN EMBRYO.
FIGURES 2.390 TO 2.392. RUDIMENTARY KIDNEYS AND SEXUAL ORGANS.
FIGURES 2.393 AND 2.394. URINARY AND SEXUAL ORGANS OF SALAMANDER.
FIGURE 2.395. PRIMITIVE KIDNEYS OF HUMAN EMBRYO.
FIGURES 2.396 TO 2.398. URINARY ORGANS OF OX-EMBRYOS.
FIGURE 2.399. SEXUAL ORGANS OF WATER-MOLE.
FIGURES 2.400 AND 2.401. ORIGINAL POSITION OF SEXUAL GLANDS.
FIGURE 2.402. UROGENITAL SYSTEM OF HUMAN EMBRYO.
FIGURE 2.403. SECTION OF OVARY.
FIGURES 2.404 TO 2.406. GRAAFIAN FOLLICLES.
FIGURE 2.407. A RIPE GRAAFIAN FOLLICLE.
FIGURE 2.408. THE HUMAN OVUM.

CHAPTER 2.
16. STRUCTURE OF THE LANCELET AND THE SEA-SQUIRT.
In turning from the embryology to the phylogeny of man--from the development of the individual to that of the species--we must bear in mind the direct causal connection that exists between these two main branches of the science of human evolution. This important causal nexus finds its simplest expression in "the fundamental law of organic development," the content and purport of which we have fully considered in the first chapter. According to this biogenetic law, ontogeny is a brief and condensed recapitulation of phylogeny. If this compendious reproduction were complete in all cases, it would be very easy to construct the whole story of evolution on an embryonic basis. When we wanted to know the ancestors of any higher organism, and, therefore, of man--to know from what forms the race as a whole has been evolved we should merely have to follow the series of forms in the development of the individual from the ovum; we could then regard each of the successive forms as the representative of an extinct ancestral form. However, this direct application of ontogenetic facts to phylogenetic ideas is possible, without limitations, only in a very small section of the animal kingdom. There are, it is true, still a number of lower invertebrates (for instance, some of the Zoophyta and Vermalia) in which we are justified in recognising at once each embryonic form as the historical reproduction, or silhouette, as it were, of an extinct ancestor. But in the great majority of the animals, and in the case of man, this is impossible, because the embryonic forms themselves have been modified through the change of the conditions of existence, and have lost their original character to some extent. During the immeasurable course of organic history, the many millions of years during which life was developing on our planet, secondary changes of the embryonic forms have taken place in most animals. The young of animals (not only detached larvae, but also the embryos enclosed in the womb) may be modified by the influence of the environment, just as well as the mature organisms are by adaptation to the conditions of life; even species are altered during the embryonic development. Moreover, it is an advantage for all higher organisms (and the advantage is greater the more advanced they are) to curtail and simplify the original course of development, and thus to obliterate the traces of their ancestors. The higher the individual organism is in the animal kingdom, the less completely does it reproduce in its embryonic development the series of its ancestors, for reasons that are as yet only partly known to us. The fact is easily proved by comparing the different developments of higher and lower animals in any single stem.
In order to appreciate this important feature, we have distributed the embryological phenomena in two groups, palingenetic and cenogenetic. Under palingenesis we count those facts of embryology that we can directly regard as a faithful synopsis of