CHAPTER XXVIII.

METHOD IN THE HISTORY OF LIFE.

NATURE has always issued her bulletins. It is a most

interesting fact in the history of the animal creation that Nature advertised her plans in the very earliest creative acts. In our study of the relics of the primeval ages we do not find the grand and fundamental purposes of Infinite Wisdom unfolding themselves by degrees as type after type of organic life made its advent upon our planet. It is quite true that the full development of Nature's schemes can only be apprehended in the ultimate results, and that, with our highest wisdom, we are continually surprised at the wealth of resources exposed in the unfolding of a simple plan. But Nature had her plans, and these were mature in the very beginning. All possible contingencies being foreseen, no amendments or modifications have been necessitated by the growth of successive populations and the march of human improvement. The outlines of Nature's grand methods were announced in her initial creative efforts. It was thus in the plan of continental development; it was thus in the plan of the animal creation. It is only in the infinite flexibility of her plans, and in the inexhaustible richness of their filling up, that Nature transcends all the possibilities of human expectation.

To the geologist no fact is more familiar or more patent than the simultaneous introduction upon the earth of three of the four fundamental plans of animal structure which in the following ages were to sport into the infinite variety

of individual forms that diversify the surface of the earth at the present day. Saying nothing about the solitary Eozoön, which stands inscrutable, isolated, and mysterious in the remote ages of Eozoic Time, like a desolate islet in the midst of a dark, and trackless, and tempest-beaten sea, we find that upon the very threshold of Paleozoic Time representatives of Radiates, Molluscs, and Articulates burst into multifarious being almost simultaneously. So nearly simultaneous was the appearance of each of these types, that all hypothesis of their genealogical succession is rationally precluded. The doctrine of development finds great discountenance in the very first of the facts from which such a doctrine ought to derive its support. Later in the history of the world Vertebrates made their advent, and thus were laid the four corner-stones on which Nature has built the superstructure of the animal creation. Among all the multitudes of organic forms which have been disentombed from the cemeteries of the solid rocks, we have found none which were not conformed to one of the four fundamental types announced in the beginning. Here is no caprice, here is no chance, but the constancy, and order, and persistence of intelligence, foresight, and fixed purpose.

When this grand procession of organic forms was marshaling for its movement through time, the Supreme Intelligence sent it forward in four columns, in each of which was dominant one of the four ideas of structure. But as Nature did not range her four columns in linear order, but set them abreast of each other, so she was equally far from bringing forward the subordinate divisions of each column or plan in any thing like a fixed progressive succession. Neither the highest and most exalted forms, nor the lowest and most humble, were ordained to take absolute precedence. In the sub-kingdom of Radiates the type was

introduced by Echinoderms, Acalephs, and Protozoans, the two highest and the lowest of the four classes. True coral animals perhaps made their appearance a little later. In the sub-kingdom of Molluscs all the classes stand abreast on their first advent; in that of Articulates, the two lower classes, Crustaceans and Worms, preceded by a long interval the Insecteans; and in the sub-kingdom of Vertebrates the classes followed each other in regular gradational succession. Thus we see that, so far as class-groups are concerned, it is impossible to reduce the order of succession to any general formula. How is it with the orders of the respective classes? Among Echinoderms, Cystideans appeared before the successively higher Crinideans, Starfishes, and Sea-urchins; among Acalephs, the horny Graptolites appeared before the Coral-builders; among Protozoans, the Sponges, which ally themselves to Polypi, appeared before the lowest types-always disregarding the mysterious Eozoön. On the whole, the order of succession among the groups, based upon relative rank, is, with Radiates, from below upward. With Molluscs we find the straight and simple Orthoceratites preceding the higher Cephalapods; the arcuate and the entire-mouthed Gasteropods leading the higher spiral and flesh-eating families; the asiphonal Lamellibranchs antedating those with more complete respiratory apparatus, and the horny-shelled Lingula and Discina, among Brachiopods, appearing before the stony-shelled and stony-armed Spirifers and Terebratulas. Among the Articulate and Vertebrate classes the gradational succession of the various orders is tolerably perfect. But I must refrain from alluding to specific facts. The following grand generalization rests on a broad survey of data upon which it would be inappropriate, in this place, to enter.

There is no successional relation between the four subkingdoms of animals, nor even between the several classes

of the invertebrate sub-kingdoms; but among the orders of the several classes and the classes of the Vertebrates we find generally a progress from lower to higher in the order of introduction.

But there is another principle, complementary to this, which needs to be united to it in order to present us with a true view of Nature's method. There has generally been a downward as well as an upward unfolding of each type from the central forms in which it was first embodied. Trilobites, the first representatives of the Crustacean type, belong indeed to the lowest group, but do not lie at the bottom of the group--the lower members, as well as the higher groups, coming into being at subsequent periods. The earliest reptiles were not the lowest of the Amphibians, but Labyrinthodonts, the highest Amphibians; and from this starting-point the reptilian type expanded both upward and downward. Vertebrates began, not with the lowest fishes, but with a grade of fishes above the mean level of the type in the possession of several reptilian characteristics. From here the type rose still higher to the strongly sauroid forms, and descended to the Teliosts, or typical fishes, with their aberrant and degraded forms-the lamprey and the lancelet. We shall arrive, therefore, at the truest expression of the plan of Nature in reference to the succession of organic beings by saying that each type was first introduced at a nodal point, from which the stream of development proceeded in both directions-the lowest forms in many instances being reached only in the modern age; so that, in some cases, after the culmination of a type, it has suffered a degeneration into the lower grades already passed.

Another fact strikes us in a review of the succession of life in past time. Life has presented itself not so much in a series of sharply-restricted organic forms, rising or de

scending in regular order, as in a succession of dominant ideas, each in its own age expressing itself in more than one organic type. Thus, in the reign of reptiles, the reptilian idea was dominant, and we find it invading the structure of the contemporaneous fishes. Afterward the avian or ornithic idea became dominant, and reptiles were endowed with wings, and even with feathers-if we may credit the reptilian character of the Archeopteryx of Solenhofen. Still later, the mammalian idea became dominant, and the forms of the Ichthyosaurus and Plesiosaurus, but more especially of the terrestrial Deinosaurs, indexed the impress of that idea upon the reptilian class. Even in the Age of Molluscs, the dominant idea was expressed in the bivalve nature of the Ostracoid Crustaceans.

The forms styled "synthetic" or "comprehensive" types may perhaps be generalized under the formula of dominant ideas. Comprehensive types are those in which certain characteristics of a group are ingrafted upon a distinct though kindred stock. The Ganoid fishes are of this kind, since they combine reptilian with fish-like features. The Labyrinthodonts were comprehensive types, because they were Amphibians with the scaly covering of Reptiles. The Lepidodendra of the Coal era combined the characteristics of the Cryptogams with the foliage and general habits of the Conifers. Such a synthesis of types seems to be occasioned by the overlapping of consecutive ideas in time-a penumbra occurring while the last dominant idea is passing under the shadow of the coming one. The Pterosaurs, or flying reptiles, were the most marvelous of all comprehensive or penumbral types. On the basis structure of a reptile we find ingrafted the head and neck of a bird, the trunk and tail of a quadruped, and the leathery wings of a bat; while, not improbably, their feet were furnished with a web; so that these creatures were fitted for all elements,