Изображения страниц

cular pleasure," says Würtenberger, "when, after divers careful comparative studies, I at last detected an interesting and simple conformity to law in the variations of the Ammonites. Namely, on the first appearance of a modification which subsequently attains essential importance in an entire group, it is only slightly indicated on a portion of the last convolution. Towards more recent deposits, this modification is more and more plainly shown, and then advances, following the spiral course of


FIG. 19. Ammonites Humphresianus. A form analogous to the Planulata.

the shell; that is to say, it gradually takes possession of the central turns also, as we trace the forms to higher strata. This reproduction in younger stages of life of modifications first occurring at a more advanced age, makes but slow progress, so that we see the older forms repeated with great persistency in the central turns. Frequently a modification of this sort has taken possession of only a small part of the convolutions, when a new one already appears at the outside, and follows the

first. Thus searching through the strata from below upwards, we see modification after modification beginning at the outer part of the Ammonites, and advancing towards the centre of the discs. The innermost convolutions often resist these innovations with great persistency, so that we usually find upon their surface several of these states of development closely compressed, as the shell of the individual Ammonite begins with the old morphological type, and then adopts the modifications in the same order in which they follow in vast periods in the geological development of the groups concerned."

"The Ammonites," he says moreover, "thus obtain at an advanced and maturer age-only when they have. gone through the development inherited from their parents, and as much as possible in the same manner as their parents the power of modifying themselves in a new direction, that is to say, of adapting themselves to new conditions; yet these modifications may then be transmitted to the offspring, so as to appear in each subsequent generation a trifle earlier, until this phase of development in its turn characterizes the greater portion of the period of growth. But this last and longest phase of development scarcely ever suffers itself to be supplanted by new ones, formed in like manner; heredity operates so powerfully, that a period of development thus once predominant, is repeated in the infancy of the Ammonites, even though but slightly indicated. Hence in an individual Ammonite from a recent stratum, the periods of development compressed and forced back upon the innermost convolutions, must appear in the same succession in which they wrested the dominion

from one another. It is extremely interesting to study the development of the Inflata of the upper white Jura, which follow the Ammonites liparus (whose externally visible convolutions display only one row of spines), and carefully break off convolution by convolution. Towards the middle there is a region in which there are always two rows of spines; nearer the centre the innermost row disappears; soon afterwards the outer one also; and the nucleus, some millimetres in diameter, now appears for about half a turn as a Planulatum, with distinct ribs, which, towards the beginning, likewise disappear. Thus even the Planulate ribs, which prevailed among the Liassic ancestors of these Inflata, and were supplanted by the spines as early as in the brown Jura, still distinguish these later and essentially modified descendants during a short period of their youth."


Würtenberger further shows how these relations can be simply explained by the Darwinian theory alone; "without it we should have only an extraordinary problem." It was natural to test the applicability of the theory of selection also on the forms allied to the Ammonites, such as the Ancyloceras; namely, the genera in which the convolutions do not touch and partially conceal one another, as in genuine Ammonites, and which, as late comers and side shoots of the group, seemed destined to decay. Selection and decay?

FIG. 20. Ancyloceras.

Würtenberger shows how the abandonment of contact


in the convolutions was to the spinous Ammonites an advantage which would be established by selection. If other paleontologists consider the fluctuations of form accompanying the relaxation of the closed spiral as evincing the decline of the group, no contradiction seems to be implied, for what was originally used as an advantage by natural selection, proved injurious in its consequences.

As we have seen, the earliest states are obliterated to

[graphic][ocr errors][ocr errors][merged small]

such a degree by curtailment of development that the indication of the nature of the progenitors continually diminishes. But our theory necessarily leads to the conviction that the families within which we have as yet been able to compare Ontogenesis with Philogenesis,

constantly approximate in their origin, and vindicate the expectation that at least here and there, in the individual development of single representatives of the various families, witnesses of their common derivation should come to light. This likewise occurs, and to such a degree that in the earliest larval stages a link is established between the lowest and the highest animals. If a number of groups of the lowest living beings, in which the various vital functions of nutrition, irritability, motion, and reproduction are supplied by amorphous protoplasm,—if these be separated, as by Haeckel, into a neutral kingdom, owing to the absence of sexual reproduction, we must likewise agree with him in attributing to the Spongiadæ ranking next to the Protista, the name of animals, on account of their sexual propagation and the nature of their embryonic development and first larval phases.

Haeckel has bestowed on one larval stage of the calcareous sponges the title of Gastrula, wherein the animal represents a sac, or, in other words, a stomach provided with a mouth-like orifice. The walls are formed of two rows of cells, the outer one consisting of ciliated cells; that is to say, each cell is furnished with a long filament. At the orifice of the sac, the outer row merges into the inner one, and from these two membranes the body of the sponge is constructed in a definite manner. Now, if this Gastrula larva reappears in the Colenterata, Polypes, and Medusa, in which the gradual development from the two membranes, the entoderm and ectoderm, into the most complex forms has long been known; and if, as Haeckel has further shown, the osculum, or larger opening of the spongiadæ may be

« ПредыдущаяПродолжить »