For a generation this truth has been accepted by biologists.*<* It was in 1852 that I became acquainted with Von Baer's expressionof this general principle. The universality of law had ever been with mea postulate, carrying with it a correlative belief, tacit if not avowed,in unity of method throughout Nature. This statement that every plant andanimal, originally homogeneous, becomes gradually heterogeneous, set up aco-ordination among thoughts which were previously unorganized, or but partiallyorganized. It is true that in Social Statics (Part IV, §§12-16),published before meeting with Von Baer's formula, the development of an individualorganism and the development of a social organism, are described as alikeconsisting in advance from simplicity to complexity, and from independentlike parts to mutually-dependent unlike parts. But though admitting of extensionto other super-organic phenomena, this statement was too special to admitof extension to inorganic phenomena. The great aid tendered by Von Baer'sformula arose from its higher abstractness; since, only when organic transformationshad been expressed in the most abstract terms, was the way opened for seeingwhat they had in common with inorganic transformations. The conviction thatthis process of change gone through by each unfolding organism, is a processgone through by all things, found its first coherent statement in an essayon "Progress: its Law and Cause" which I published in the WestminsterReview for April, 1857 -- an essay with the first half of which thischapter coincides in substance, and partly in form. In that essay, how ever,as also in the first edition of this work, I fell into the error of supposingthat the transformation of the homogeneous into the heterogeneous constitutesEvolution. We have seen that this is not so. It constitutes the secondaryre-distribution accompanying the primary re-distribution in that Evolutionwhich we distinguish as compound; or rather is we shill presently see, itconstitutes the most conspicuous trait of this secondary re-distribution.> §120. When we pass from individual forms of life to life at large,and ask whether the same law is seen in the ensemble of its manifestations-- whether modern plants and animals have more heterogeneous structures thanancient ones, and whether the Earth's present Flora and Fauna are more heterogeneousthan the Flora and Fauna of the past, -- we find the evidence so fragmentarythat nearly every conclusion is open to dispute. Three-fifths of the Earth'ssurface being covered by water; a great part of the exposed land being inaccessibleto, or untravelled by, the geologist; the most of the remainder having beenscarcely more than glanced at; and even familiar portions, as England, havingbeen so imperfectly explored that a new series of strata has been added withinthese few years; it is clearly impossible to say with any certainty whatcreatures have, and what have not, existed at any particular period. Consideringthe perishable nature of many of the lower organic forms, the metamorphosisof many beds of sediment, and the gaps that occur among the rest, we shallsee further reason for distrusting our deductions. On the one hand, the repeateddiscovery of vertebrate remains in strata previously supposed to containnone -- of reptiles where only fish were thought to exist, and of mammalswhere it was believed there were no creatures higher than reptiles; rendersit daily more manifest how small is the value of negative evidence. On theother hand, the worthlessness of the assumption that we have found the earliest,or anything like the earliest, organic remains, is becoming equally clear.
That the oldest known aqueous formations have been greatly changed by igneousaction, and that still older ones have been totally transformed by it, isbecoming undeniable. And the fact that sedimentary strata earlier than anywe know have been melted up, being admitted, it must also be admitted thatwe cannot say how far back in time this destruction of sedimentary stratahas been going on. For aught we know to the contrary, only the last chaptersof the Earth's biological history may have come down to us.
