A belief in abiogenesis, it is argued. is a necessary corollary from the doctrine of evolution. This, says Huxley, may be true of the occurrence of abiogenesis at some time; but if the present day, or anv recorded epoch of geological time, be a question, the exact contrary holds good. If all living beings have been evolved from pre-existing forms of life, it is enough that a single particle of living protoplasm should once have appeared on the globe, as the result of whatever agency. In the eyes of a consistent evolutionist any further independent formation of protoplasm would be sheer waste. The production of living matter since the time of its first appearance, only by way of bio genesia, implies that the specific forms of the lower kinds of life have undergone but little change in the course of :geological time, and this is said to be inconsistent with the doctrine of evolution. But, in tlic first place, the fact is not inconsistent with the doc trine of evolution properly understood, that doctrine being perfectly consistent with either the progression, the retrogression, or the stationary condition of any particular species for indefinite periods of time; and, secondly, if it were, it would be so much the worse for the doctrine of evolution, inasmuch as it is unquestionably true that certain even highly organized forms of life have persisted without any sensible change for very long periods. The fact is. says Huxley, that at the present moment there is not a shadow of trustworthy direct evidence that a biogenesis does take place within the period during which the existenceof life on the globeis recorded. But it need hardly be pointed out that this fact does not in the slightest degree interfere with any conclusion that may be arrived at deductively from other considerations that, at some time or other, abiogenesis must have taken place. If the hypothesis of evolution be true, living matter must have arisen from, or, at least, in place of lion-living matter; for by the hypothesis, the condition of the globe was at one time such that living matter could not have existed in it, life being entirely incompatible with the gaseous state. But living matter once originated, there is no necessity for other origination, since the hypothesis postulates the unlimited, though perhaps not illimitable, modifiability of such matter.
Of the causes which have led to the origination of living matter, or the origin of the species, it may be said (continues Huxley) that we know absolutely nothing. But, pos tulating the existence of living matter endowed with that power of hereditary trans mission and that tendency to vary which is found in all such matter, 3Ir. Darwin has declared that the interaction between living matter and surrounding conditions, which results in the survival of the fittest, is sufficient to account for the gradual evolution of plants and animals from their simplest to their most complicated forms, and for the known phenomena of morphology, physiology, and distribution. While much weight
is to be conceded to the evidences for the conceivable sufficiency of the above interaction for the alleged results, its actual efficiency in the history of the case must be regarded as at present only a hypothesis.
If all living beings have come into existence by the gradual modification, through a long series of generations, of a primordial living matter, the phenomena of embryonic development ought to be explicable as particular cases of the general law of hereditary transmission. On this view, a tadpole is first a fish and then a tailed amphibian, pro vided with both gills and lungs, before it becomes a frog, because the frog was the last term in a series of modifications whereby some ancient fish became an urodele amphib ian, and the urodele amphibian became an anurous amphibian. In fact, the develop ment of the embryo is a recapitulation of the ancestral history of the species. If this be so, it follows that the development of any organism should furnish the key to its ances tral history; and the attempt to decipher the full pedigree of organisms from so much of the family history as is recorded in their development has given rise to a special branch of biological speculation termed phylugeny. In practice, however, the reconstruction of the pedigree of a group from the developmental history of its existing members is fraught with difficulties. It is highly probable that the series of developmental stages of the individual organism never presents more than, an abbreviated and condensed sum mary of ancestral conditions; while this summary is often strangely modified by varia tion and adaptation to conditions; and it must be confessed that in most cases we can do little better than guess at what is genuine recapitulation of ancestral forms, and what is the effect of comparatively late adaptation. The only perfectly safe foundation for the doctrine of evolution (concludes Huxley) lies in the historical or, rather, archmological evidence that particular organisms have arisen by the gradual modification of their pre decessors, which is furnished by fossil remains. That evidence is daily increasing in amount and in weight; and it is to be hoped that the comparison of the actual pedigree of these organisms with the phenomena of their development may furnish some criterion by which the validity of phylogenetic conclusions, deduced from the facts'of embryology alone, may he satisfactorily tested. According to this statement of the case by Huxley, it would follow that the doctrine of evolution is, for the present, to be held in expec tancy; and that it is possible to use concerning it terms of too positive assertion.