Peter Chalmers Mitchell

“ Evolution, or development, is, in fact, at present employed in biology as a general name for the history of the steps by which any living being has acquired the morphological and the physiological characters which distinguish it. As civil history may be divided into biography, which is the history of individuals, and universal history, which is the history of the human race, so evolution falls naturally into two categories—the evolution of the individual (see Embryology) and the evolution of the sum of living beings. ”

“ LONGEVITY, a term applied to express either the length or the duration of life in any organism, but, as cases of long duration excite most interest, frequently used to denote a relatively unusual prolongation of life. There is no reason to suppose that protoplasm, the living material of organisms, has a necessarily limited duration of life, provided that the conditions proper to it are maintained, and it has been argued that since every living organism comes into existence as a piece of the protoplasm of a pre-existing living organism, protoplasm is potentially immortal. ”

“ With regard to adaptations, it is becoming more and more apparent, as experimental knowledge advances, that it is a fundamental property of every living organism in every stage of its existence to display adaptive response to its environment. To what extent such responses are transmitted to offspring, and what part they play in the formation of the adaptive characters that are conspicuous in many animals, remain dubious, but it is at least clear that natural selection can favour those individuals and those races which show the greatest power of responsive plasticity in the individual. ”

“ Notwithstanding the elaborate disguise which fear of the powers that were led Descartes to throw over his real opinions, it is impossible to read the Principes de la philosophie without acquiring the conviction that this great philosopher held that the physical world and all things in it, whether living or not living, have originated by a process of evolution, due to the continuous operation of purely physical causes, out of a primitive relatively formless matter. ”

“ The objective of a propaganda is to promote the interests of those who contrive it, rather than to benefit those to whom it is addressed; in advertisement to sell an article; in publicity to state a case; in politics to forward a policy; in war to bring victory. This differentiates it from the diffusion of useful knowledge; the evangel of a mission ”

“ The normal specific longevity, the age to which all normal individuals of a species would survive under the most favourable conditions, must depend on constitution and structure. No doubt selection is involved, as it is obvious that creatures would perish if their constitution and structure were not such that they could live long enough to reproduce their kind. The direct explanation, however, must be sought for in size, complexity of structure, length of period of growth, capacity to withstand the wear and tear of life and such other intrinsic qualities. ”

“ De la Mettrie worked out a materialistic doctrine of the origin of things, according to which sensation and consciousness are nothing but a development out of matter. He sought (L’Homme-machine) to connect man in his original condition with the lower animals, and emphasized (L’Homme-plante) the essential unity of plan of all living things. Helvétius, in his work on man, referred all differences between our species and the lower animals to certain peculiarities of organization, and so prepared the way for a conception of human development out of lower forms as a process of physical evolution. ”

“ VARIATION AND SELECTION, in biology. Since the publication in 1859 of Charles Darwin's Origin of Species, the theory of evolution of animals and plants (see Evolution) has rested on a linking of the conceptions of variation and selection. Living organisms vary, that is to say, no two individuals are exactly alike; the death-rate and the multiplication-rate are to a certain extent selective, that is to say, on the average, in the long run, they favour certain variations and oppress other variations. Co-operation of the two factors appears to supply a causal theory of the occurrence of evolution ”

“ The methods of a propaganda are limited only by the resources and the ingenuity of its promoters. They may be studied in their most intensive form in the propagandist efforts during a war; the magnitude of the object secures the necessary funds, and at the same time attracts the services of persons of more intellect and character than would usually devote themselves to such a pursuit; in the atmosphere of war, moreover, truth, like many other fine qualities of humanity, is judged by expediency, with varying answers. ”

“ PARASITISM, in biology, the condition of an organism which obtains its nourishment wholly or partially from the body of another living organism, and which usually brings about extensive modifications in both guest and host, a phenomenon widespread amongst animals and plants. ”

“ The whole world is represented by the figure of a tree, of which the seeds and roots are the first indeterminate matter, the leaves the accidents, the twigs and branches corruptible creatures, the blossoms the rational soul, and the fruit pure spirits or angels. It is also described as a bifurcation of two twigs, mental and bodily creation out of a common root. One might almost say that Duns Scotus recognizes the principle of a gradual physical evolution, only that he chooses to represent the mechanism by which the process is brought about by means of quaint scholastic fictions. ”

“ It is understood that the term reproductive activity covers not merely the production of new individuals but the care of these by the parents until they are self-sufficient. The average longevity, according to Weismann, is adapted to the needs of the species; it is sufficiently long to secure that the requisite number of new individuals is produced and protected. He has brought together a large number of instances which show that there is a relation between duration of life and fertility. ”

“ Empirical Study of Heredity.—The fundamental basis of heredity is the separation of a mass from the parent (germ-plasm) which under certain conditions grows into an individual resembling the parent. The goal of the study of heredity will be reached only when all the phenomena can be referred to the nature of the germ-plasm and of its relations to the conditions under which it grows, but we have seen how far our knowledge is from any attempt at such references. ”

“ Quoique mon unique objet dans cet article n’ait été que de traiter de la cause physique de l’entretien de la vie des êtres organiques, malgré cela j’ai osé avancer en débutant, que l’existence de ces êtres étonnants n’appartiennent nullement à la nature; que tout ce qu’on peut entendre par le mot nature, ne pouvoit donner la vie, c’est-à-dire, que toutes les qualités de la matière, jointes à toutes les circonstances possibles, et même à l’activité répandue dans l’univers, ne pouvaient point produire un être muni du mouvement organique, capable de reproduire son semblable, et sujet à la mort. ”

“ Origin of Parasitism.—Now that the theory of spontaneous generation has been disproved, the problem of parasitism is no more than detection of the various causes which may have led organisms to change their environment. Every kind of parasite has relations more or less closely akin which have not acquired the parasitic habit, and every gradation exists between temporary and permanent parasites, between creatures that have been only slightly modified and those that have been profoundly modified in relation to this habit. ”

“ Their importance in the theory of evolution is none the less very great. In the first place, they lessen the number of separate facts to be explained; in the second, they limit the field within which explanation must be sought, since, for instance, if a particular mode of repetition of parts occur in mosses, in flowering-plants, in beetles and in elephants, the seeker of ultimate explanations may exclude from the field of his inquiry all the conditions individual to these different organic forms, and confine himself only to what is common to all of them ”