August 2, 1906] 



NA TURE 



327 



ing outgrowth of the fcEtus called the placenta fairly well 

 developed, so that they must be regarded as a degenerate 

 side-branch of the placental mammals, and not as primitive 

 forerunners of that dominant series. 



Speculations as to the ancestral connection of the great 

 group of vertebrates with other great groups have been 

 varied and ingenious ; but most naturalists are now in- 

 clined to the view that it is a mistake to assume any such 

 connection in the case of vertebrates of a more definite 

 character than we admit in the case of starfishes, shell- 

 fish, and insects. All these groups are ultimately connected 

 by very simple, remote, and not by pro.ximate ancestors, 

 with one another and with the ancestors of vertebrates. 



The origin of the limbs of vertebrates is now generally 

 agreed to be correctly indicated in the Thatcher-Mivarl- 

 Balfour theory to the effect that they are derived from a 

 pair of continuous lateral iins, in fish-like ancestors, similar 

 in every way to the continuous median dorsal fin of fishes. , 



The discovery of the formation of true spermatozoa by 

 simple unicellular animals of the group Protozoa is a start- 

 ling thing, for it had always been supposed that these 

 peculiar reproductive elements were only formed by multi- 

 cellular organisms. They have been discovered in some of 

 the grcgarina-Iike animalcules, the Coccidia, and also in 

 the blood-parasites. 



Among plants one of the most important discoveries 

 relates to these same reproductive elements, the 

 spermatozoa, which by botanists are called antherozoids. 

 .•\ great difference between the whole higher series of plants, 

 the flowering plants or phanerogams, and the cryptogams 

 or lower plants, including ferns, mosses, and algae, was 

 held to be that the latter produce vibratile spermatozoa 

 like those of animals which swim in liquid and fertilise 

 the motionless egg-cell of the plant. Two Japanese 

 l)Otanists (and the origin of this discovery from Japan, 

 from the University of Tokio, in itself marks an era in 

 the history of science), Hirase and Ikeno, astonished the 

 botanical world fifteen years ago by showing that motile 

 antherozoids or spermatozoa are produced by two gymno- 

 sperms, the ginkgo tree (or Salisburya) and the cycads. 

 The pollen-tube, which is the fertilising agent in all other 

 phanerogams, develops in these cone-bearing trees, 

 beautiful motile spermatozoa, which swim in a cup of 

 liquid provided for them in connection with the ovules. 

 Thus a great distinction between phanerogams and crypto- 

 gams was broken down, and the actual nature of the pollen- 

 tube as a potential parent of spermatozoids demonstrated. 



When we come to the results of the digging out and 

 study of extinct plants and animals, the most remarkable 

 results of all in regard to the atTinities and pedigree of 

 organisms have been obtained, .'\mong plants the transition 

 between cryptogams and phanerogams has been practically 

 bridged over by the discovery that certain fern-like plants 

 of the Coal-measures — the Cycadofilices, supposed to be 

 true ferns, are really seed-bearing plants and not ferns at 

 all, but phanerogams of a primitive type, allied to the 

 cycads and gymnosperms. They have been re-christened 

 Pteridosperms by Scott, who, together with F. Oliver and 

 Seward, has been the chief discoverer in this most interest- 

 ing field. 



By their fossil remains whole series of new genera of 

 extinct mammals have been traced through the Tertiary 

 strata of North America and their genetic connections 

 established ; and from yet older strata of the same prolific 

 source we have almost complete knowledge of several 

 genera of huge extinct Dinosauria of great variety of form 

 and habit. 



The discoveries by Seeley at the Cape, and by Amalitzky 

 in North Russia of identical genera of Triassic reptiles, 

 which in many respects resemble the Mammalia and con- 

 stitute the group Theromorpha, is also a prominent feature 

 in the paljcontology of the past twenty-five years. Nor 

 must we forget the extraordinary Silurian fishes discovered 

 and described in Scotland by Prof. Traquair. The most 

 important discovery of the kind of late years has been 

 that of the Upper Eocene and Miocene mammals of the 

 Egyptian Fayum, excavated by the Egyptian Geological 

 Survey and by Dr. .Andrews of the Natural History 

 Museum, who has described and figured the remains. 

 They include a huge four-horned animal as big as a 



NO. I918, VOL. 74] 



rhinoceros, but quite peculiar in its characters — the 

 Arsinoilherium — and the ancestors of the elephants, a 

 group which was abundant in Miocene and Pliocene times 

 in Europe and Asia, and in still later times in America, 

 and survives at tlie present day in its representatives the 

 African and Indian elephants. One of the European extinct 

 elephants— the Tetrabelodon — had, we have long known, 

 an immensely long lower jaw with large chisel-shaped 

 terminal teeth. It had been suggested by me that the 

 modern elephant's trunk must have been derived from the 

 soft upper jaw and nasal area, which rested on this 

 elongated lower jaw, by the shortening (in the course of 

 natural selection and modification by descent) of this long 

 lower jaw, to the present small dimensions of the elephant's 

 lower jaw, and the consequent down-dropping of the un- 

 shortened upper jaw and lips, which thus become the 

 proboscis. Dr. Andrews has described from Egypt and 

 placed in the Museum in London specimens of two new 

 genera — one, Pala;omastodon, in which there is a long, 

 powerful jaw, an elongated face, and an increased number 

 of molar teeth; the second, Mceritherium, an animal with 

 a hippopotamus-like head, comparatively minute tusks, and 

 a well-developed complement of incisor, canine, and molar 

 teeth, like a typical ungulate mammal. Undoubtedly we 

 have in these two forms the indications of the steps by 

 which the elephants have been evolved from ordinary- 

 looking pig-like creatures of moderate size, devoid of trunk 

 or tusks. Other remains belonging to this great mid- 

 African Eocene fauna indicate that not only the Elephants 

 but the Sirenia took their origin in this area. Amongst 

 them are also gigantic forms of Hyrax, like the little 

 Syrian coney and many other new mammals and reptiles. 



Another great area of exploration and source of new 

 things has been the southern part of Argentina and Pata- 

 gonia, where Ameghino, Moreno, and Scott of Princeton 

 have brought to light a wonderful series of extinct ant- 

 eaters, armadilloes, huge sloths, and strange ungulates, 

 reaching back into early Tertiary times. But most re- 

 markable has been the discovery in this area of remains 

 which indicate a former connection with the Australian land 

 surface. This connection is suggested by the discovery in 

 the Santa Cruz strata, considered to be of early Tertiary 

 date, of remains of a huge horned tortoise which is 

 generically identical with one found fossil in the Australian 

 area of later date, and known as Miolania. In the same 

 wonderful area we have the discovery in a cave of the 

 fresh bones, hairy skin, and dung of animals supposed to 

 be extinct, viz. the giant sloth, Mylodon, and the peculiar 

 horse, Onohippidium. These remains seem to belong to 

 survivors from the last submergence of this strangely 

 mobile land-surface, and it is not improbable that sorne 

 individuals of this " extinct " fauna are still living in 

 Patagonia. The region is still unexplored, and those who 

 set out to examine it have, by some strange fatality, 

 hitherto failed to carry out the professed purpose of their 

 expeditions. 



I cannot quit this immense field of gathered fact and 

 growing generalisation without alluding to the study of 

 animal embryology and the germ-layer theory, which has 

 to some extent been superseded by the study of embryonic 

 cell-lineage, so well pursued by some American micro- 

 scopists. The great generalisation of the study of the 

 germ-layers and their formation seems to be now firmly 

 established — namely, that the earliest multicellular animals 

 were possessed of one structural cavity, the enteron, 

 surrounded by a double layer of cells, the ectoderm and 

 endoderm. These Enterocoela or Ccelentera gave rise to 

 forms having a second great body-cavity, the ccelom, which 

 originated not as a split between the two layers, as was 

 supposed twenty-five years ago by Haeckel and Gegenbaur 

 and their pupils, but by a pouching of the enteron to form 

 one or more cavities in which the reproductive cells should 

 develop — pouchings which became nipped off from the 

 cavity of their origin, and formed thus the independent 

 coelom. The animals so provided are the Ccelomocosla 

 (as opposed to the Enterocoela), and comprise all animals 

 above the polyps, jelly-fish, corals, and sea-anemones. It 

 has been established in these twenty-five years that the 

 coelom is a definite structural unit of the higher groups, 

 and that outgrowths from it to the exterior (coelomoducts) 



