to the conditions of environment can be tested. It is reasonable to expect, therefore, that all modifications of organic structure, which imply strictly intrinsic differentiation of the co-ordinated structure and function of the organism, were evolved by processes vastly more rapid than those of the extrinsic modification of structures already present in the race.

We have seen how Brachiopods furnish us with the data with which to trace the laws of the historical evolution of the more important characters exhibited by any particular Brachiopod. These characters have fallen into natural divisions, or groups of various rank, which are scientifically recognized as class, ordinal, subordinal, etc., characters. We have seen how the characters which we call subordinal, when they are arranged in the order of their morphological affinities, present a series of forms whose elaboration is as complete by the beginning of the Upper Silurian as it was at any later time; therefore we drew the conclusion that so far as the subordinal characters and those of higher rank are concerned, the differentiation expressed by these characters took place in the lower half of the Paleozoic time. As far as the facts are in evidence, we find that the characters of this kind were rapidly introduced: rapidly in relation to the degree of differentiation indicated by the characters, and rapidly in comparison with the length of time they persist without apparent modification. As two ontogenetic forces are at work in the growth of the individual, to which respectively we apply the terms heredity and variability, so we recognize upon analysis of the facts of the phylogeny two kinds of evolution: (I) a progressive evolution which operates from within and is associated with pre-existing conditions; this is called intrinsic evolution; (II) another kind of evolution, observed to be more intimately co-ordinate with external conditions, which may be regarded as fundamentally a process of adjustment or adaptation of the organism to its external environment; and this is extrinsic evolution.

In the ontogenetic development of the individual there is a rapid elaboration of those typical features of the organism which express its class, ordinal, and subordinal characters, the whole framework and plan of structure being elaborated

before the individual comes into contact with external environment, while it is out of reach, so to speak, of the contests which are called struggle for existence. It is conceived that there were in like manner in evolution intrinsic modifications of internal structure, requiring for their functional operation adjustments of the whole mechanism of the body, and that these operations were relatively rapid, because they were the expression of evolutional force working from within, and in the determination of which the local and immediate conditions of environment had little or no part. As, for instance, in the plant, the special modification of ordinary tissues to produce the flower, and its complication of floral parts, relatively to the life-history of the plant is rapid, and the opening of the flower may in some sense be said to be occasioned by heat, sunshine, or, in general, by external conditions; but in a much more important sense it is true that the production of the flower is intrinsic, and is determined by ancestral, preexisting conditions, and not by those present only at the time of flowering.

CHAPTER XVI.

THE MODIFICATION OF GENERIC CHARACTERS, OR GENERIC LIFE-HISTORY.

IN the last chapter the conclusion was reached that evolution, which is the acquirement by organisms in the course of individual growth of characters not previously appearing in their ancestors, may be distinguished as of two kinds: one intrinsic, and expressing steps of progress in the differentiation of function and organization of the organism as a whole, working from within outward; the second extrinsic in nature, and expressed in the modification or adjustment of characters already differentiated to local and immediate conditions of environment.

We observed that as the particular characters examined are of higher and higher rank in classification they are more intensely intrinsic in nature, not only now, but were so in the earliest organisms of which we have any knowledge. And still further, that these more essential characters were earlier evolved, and the evidence seems to prove beyond doubt that their evolution was by steps more rapid than would be inferred from the relatively slow progress in the succession of the lesser characters, generic and specific.

Having noted the general laws of evolution respecting the more important characters of each individual, we next turn to an examination of the laws of evolution of the less important generic characters.

In the generic characters there appears to have been a rapid attainment of the total limit of modification expressed anywhere in the family, with a long persistence of the more widely divergent characters. When we examine the specific and varietal characters we observe a much slower rate of modification in individual race-series, but even here a re

markable degree of expansion of the main features of the variable characters appears very early in the history of each genus.

As an illustration of the rapid appearance of the full quota of extrinsic modifications of a new intrinsic element of structure we may examine the history of the spiral brachial appendages in the suborder Helicopegmata.

Statistics of the Life-history of the Spire-bearing Brachiopods (Helicopegmata).-The earliest trace of the spire-bearing Brachiopods is in the Ordovician, in a single simple form, the genus Zygospira.

At the next faunal stage, the base of the Upper Silurian system, there were representatives of each of the families into which the known Helicopegmata are divided (Atrypidæ, Spiriferidæ, and Athyrida); and of the twelve subfamilies into which the seventy-three recognized genera are distributed, nine are also known from as early a stage as the Upper Silurian (viz., Zygospirinæ, Dayinæ, Atrypinæ, Suessiinæ, Trigonotretinæ, Rhynchospirinæ, Hindellinæ, Athyrinæ, and Meristellina). Of the others, Uncitinæ, first appearing in the Devonian, has the same kind of brachidium as the subfamily Suessiinæ; and the loop of Diplospirinæ, appearing first in Kayseria of the Devonian and having several genera in the Triassic, is rather to be considered as an extreme differentiation of the Athyroid type; and so far as the brachidium is concerned, Koninckinina of the Mesozoic is also an extreme differentiation of the same Paleozoic type. *

The Rapid Appearance of the Different Modifications of the Brachidium. For the present discussion it matters not whether the calcification of the spirally-terminated brachidium of the Helicopegmata is a modification of that seen in the loop of the Ancylobrachia, or whether it arose from a form in which there was no calcified support; for both of the suborders, so far as evidence is at hand to show, first appeared in the Ordovician.

One intrinsic character distinguishing these suborders from all the previously existing Brachiopods is found in the presence

* In this discussion I have followed Schuchert's A Revised Classification of the Spire-bearing Brachiopoda," Am. Geol, vol. XIII. p. 102, etc., Feb. 1894.

in the former of the calcified supports, the brachidium, and it is the sudden or rapid appearance of modifications of structure of this brachidium which is under discussion.

TABLE SHOWING THE TAXONOMIC RELATIONS OF THE HELICOPEGMATA. Branch: MOLLUSCOIDEA

The above table is given to show the method of selection of this particular group of Helicopegmata for study. All the differentiation represented by the characters distinguishing the particular class, subclass, order, and suborder must be supposed to have already arisen before family characters of this particular suborder could take place.

I have adopted Dr. Beecher's ordinal classification, and take the order Telotremata, which appears to be the most fully differentiated of the orders of Brachiopods. The distinctive characters are found in the degree of differentiation of the delthyrium, or pedicle opening, and its covering, and of the brachidium or arm support. ("Pedicle opening shared by both valves in nepionic stages, usually confined to one valve in later stages, and becoming more or less limited by two deltidial plates in ephibolic stages. Arms supported by calcareous crura, spirals, or loops.") The distinctive ordinal characters I have italicized in this definition.*

It is within this order that we find the forms with special calcified parts called deltidial plates, crura, and brachidium, either loops or spirals. The subordinal distinctions are based upon the degrees and mode of elaboration of the brachial supports.

Rostracea is a new ordinal name proposed by Shuchert for the family Rhynchonellidæ of Gray, somewhat emended. It includes the genera with rostrate shells, no spondylium, and the presence of crura.

The Helicopegmata is the group proposed by Waagen to include the genera with two, calcareous, simple or double, spirally enrolled brachial supports, which may or may not be attached to each other by a variously constructed band or "loop."

The third suborder is Gray's Ancylobrachia, slightly emended by

* Beecher," Development of the Brachiopoda," Pt. 1. Am. Jour. Sci., vol. XLI. P. 355, 1891.