become organised; in all the races that survived there would be a more or less complete adaptation to the new conditions." "' 1 The inference is forced upon us that "structural changes are the slowly accumulated results of functional changes."

But what is the meaning of all these rearrangements? In what direction do they tend? Evidently, towards more and more perfect harmony and concilience. "That continual division and subdivision of forces, which is instrumental in changing the uniform into the multiform and the multiform into the more multiform," must be perpetually dissipating force, "and that dissipation, continuing as long as there remains any force unbalanced by an opposing force, must end in rest." 2 Meanwhile we have states of transitional equilibration. "These moving equilibria have a certain self-conserving power, shown in the neutralisation of perturbations and the adjustment to new conditions."

All this, you will perceive, is nothing but Hume in modern costume. "The original force still remaining in activity gives a perpetual restlessness to matter, till at last some order which can support and maintain itself is fallen upon," and this has "all the appearance of art and contrivance which we observe at present." With these essential principles to build upon, we may select a few instances of the use Mr. Spencer makes of them in interpreting the facts of biology.

"3

The first condition of evolution in general was said to be mobility; and in dealing with organic matter the first thing to be observed is "that organic bodies which exhibit the phenomena of evolution in so high a degree are mainly composed of ultimate units having extreme mobility." Of the four chief elements of organic substances, hydrogen, carbon, and nitrogen "have affinities which are narrow in their range and low in their intensity." They have but 2 Page 495. 3 The Principles of Biology, vol. i. p. 3.

1 Page 402.

slight tendency to combine with other elements. This chemical indifference, and the feeble bond of union possessed by their compounds, afford the primary conditions essential to redistribution.

Were the compounds which enter into living bodies possessed of excessive molecular mobility, no high degree of organisation could either be reached or sustained: but it so happens that "among those elements out of which living bodies are built there is an unusual tendency to unite in multiples, &c.," 1 and the groups of products thus formed supply exactly what is wanted. "We have in these colloids, of which organisms are mainly composed, just the required compromise between fluidity and solidity."

Amongst the most familiar causes of redistribution and of vital changes, none is more direct than heat. "Heat, or a raised state of molecular vibration, enables incident forces more easily to produce changes of molecular arrangement in organic matter." Every one has noticed, the effect of the sun upon plants exposed to its rays; but the evaporation to which this effect is due, by emptying the sap vessels, is the necessary condition under which fresh material is imbibed and the growth and life of the plant is continued. The exhalations of vapour through the skin and lungs of terrestrial animals, also aided by heat, prepares the way in like manner for the circulation of fresh nutritive fluids.

Light is another active instrument in the process of development. As pointed out later on, the exhalation of oxygen and the absorption of carbon in plants is due to the influence of the solar rays. "And on certain eyeless creatures that are semi-transparent, the light, permeating their substance, works some effect evinced by movement.”2

Chemical action is of chief importance in effecting changes in organic matter. In "nutrition and respiration we have the means by which the supply of materials for this active molecular rearrangement is maintained." 2 Page 28.

1 Page 23.

The one truth to be noted is that, organic matter is specially sensitive to the influences of surrounding forces; hence, specially possessed of that modifiability which is the first requisite of evolution.

Although we are here dealing with the data only of biology, we are now arrested by a point of the highest interest. We have spoken of forces acting on organic matter. It has again to be observed that the matter also reacts on the forces. Heat, light, electricity, and chemical affinity produce molecular excitement. But "this raised state of molecular vibration is itself a continuous consequence of the continuous molecular redistributions it facilitates." For instance, "the heat produced by oxidation . . . maintains the temperature at which the unoxidised portions can be readily oxidised." But besides these known modes of motion, there is "an unknown one . . . which cannot be assimilated with any otherwise recognised class; I allude to what is called nerve-force." Incident forces of every kind "produce in all creatures that have nervous systems certain nervous disturbances -disturbances which, as in ourselves, are either communicated to the chief nervous centre, and there constitute consciousness, or else result in merely physical processes that are set going elsewhere in the organism." The proofs of this fact are matters of hourly observation. Nourishment increases the supply of nerve-force, while the excessive expenditure of this force involves excessive waste of tissue. From all sides we have evidence "that nerve-force is liberated by whatever disturbs the molecular equilibrium of nerve-substance."

The possibility of vital phenomena depends, however, not merely on molecular, but more especially on sensible motion. And this form of reaction is exhibited both by plants and animals. The sap circulates: pistils and stamens are spontaneously brought into contact: the zoospores of many algæ are actively locomotive: many

1 Page 49.

small plants display a similar power, equal in fact to that of the simplest animals. The sensitive plant, the insect traps, and the fertilisation of orchids described by Darwin, are instances present to every one.

Thus it is that the changes produced by incident forces. on the atoms of organic matter become in turn the antecedents of large amounts of new modes of force, which "may be manifested as heat, light, electricity, nerve-force, or mechanical motion, according as the conditions determine."

In

The evolution of nerve - force in conjunction with mechanical motion presents us with the characteristic phenomena of life. Also, as the radical tendency of evolution is ever towards equilibration, its highest form must evince this tendency in the highest degree. accordance with this idea, we have life defined by Mr. Spencer as "the continuous adjustment of internal relations to external relations;" the most complete and perfect life being that which most completely fulfils this condition.

Whether, then, we contemplate the phenomena manifested by organic matter as a rearrangement of parts, or as new combinations of force, the interpretation of the process always at work is discoverable in the establishment of a balance between inner and outer changes; and "the final structural arrangements must be such as will meet all the forces acting on the aggregate by equivalent antagonistic forces." The question whether such structural arrangements are affected by acts of special creation, or whether they are explicable upon fundamental principles or laws of nature, such as we have been considering, can only be entertained when we are duly possessed of the arguments for and against transmutation. This brings us to the "Theory of Descent," which will be entered upon in the following letter.

LETTER IV.

THE doctrine of Evolution when applied to living beings affirms that, these are all descended from one or from a very few parent forms,1 and that the origin of new forms is to be explained by reference to mechanical laws not necessarily directed by intelligence.

Strictly speaking, the doctrine involves belief in the evolution of life from inorganic materials by a similar natural process. But on this point there is difference of opinion. Darwin, for instance, holds that “the birth, both of the species and of the individual are equally parts of that grand sequence of events which our minds refuse to accept as the result of blind chance. The understanding revolts at such a conclusion," &c.2 True, such a view might not be incompatible with the opinion that all the phenomena of Nature are to be interpreted naturally, that is to say, without reference, at any stage of their sequence, to miraculous interference. But elsewhere Darwin speaks more explicitly of life "with its several powers having been originally breathed by the Creator into a few forms or into one," &c.3 Heterogenesis, or spontaneous generation, therefore, is not an acknowledged canon of Darwinism. I propose, however, to return to the subject after a word or two on the history of the recent aspect of Evolution generally.

The tendency to classify is inherent to intelligent beings.

1 "I believe that animals have descended from at most only four or five progenitors, and plants from an equal or lesser number."-Dar

win, Origin of Species p. 518.

2 The Descent of Man, vol. ii. p.

396.

3 The Origin of Species, p. 525.