The science of life : fully illustrated in tone and line and including many diagrams

seeds lie immobile for long periods, but sooner or later they will thrust and stir. Life may move as swiftly as a flying bird, or as slowly as an expanding turnip, but it moves. It moves in response to an inner impulse. It may be stimulated to move, but the driving-force is within. It does not move simply like dust before the wind or sand stirred up by the waves.

And not only does it move of itself, but it feeds. It takes up matter from without into itself, it changes that matter chemically and from these changes it gathers the energy for movement. Crystals, stalactites, and other non-living things grow, but only by additions, by the laying-on or fitting-in of congenial particles, without any change of chemical nature or release of energy. ‘This process of taking in, assimilating and using matter, is called metabolism. Metabolism and spontaneous movement are the primary characteristics of living things.

In addition, life seems always to be produced by pre-existing lite. It presents itself as a multitude of individuals which have been produced by division or the detachment of parts from other individuals, and most of which will in their time give rise to another generation. ‘This existence in the form of distinct individuals which directly or indirectly reproduce their kind by a sort of inherent necessity is a third distinction between living and non-living things. Waves in water or wind-ripples in sand may be said to reproduce themselves, but not by a detachment and growth of their own substance: drops of oil or water grow and break up under suitable conditions, but not through any innate disposition to do so. Living things display an impulse to reproduce themselves even in adverse circumstances. So far as our knowledge goes, life arises always and only from preceding life. In the past it was believed that there could be a “spontaneous generation” of living things. Aristotle taught that plant-lice arise from the dew which falls upon plants, and that fleas spring from putrid matter.

Alexander Ross, a charmingly assured writer of the seventeenth century, in sound controversial style, reproved Sir Thomas Browne for doubting this. ‘‘So may he,” said Ross, “ doubt whether in cheese and timber worms are generated ; or if beetles and wasps in cows’ dung ; or if butterflies, locusts, grasshoppers, shell-fish, snails, eels, and such like be procreated of putrefied matter, which is apt to receive the form of that creature to which it is by formative

INTRODUCTION

power disposed. To question this is to question reason, sense, and experience. If he doubts of this, let him go to Egypt, and there he will find the fields swarming with mice, begot of the mud of Nylus to the great calamity of the inhabitants.”

Very gradually this error was dispelled. The mice story evaporated—the necessary connection of flies and maggots was demonstrated. It was only in the middle nineteenth century that the concluding dispute took place over Bastian’s assertion that abundant bacterial life appeared in sterilized infusions of hay and other material. Pasteur and others demonstrated the insufficiency of his sterilization. Germs and protected spores had escaped his boiling. It is accepted now by all biologists of repute that life arises from life and in no other way—omne vivum ex vivo. Life as we know it flows in a strictly defined stream from its remote and unknown origins, it dissolves and assimilates food, but it receives no living tributaries.

Yet the distinction of what is living and what is not living is by no means easy. Seeds, small worms and microscopic animalcula can be dried up and left totally inert for long periods of time, so that it is impossible to distinguish them from dead organisms ; then at the touch of moisture they will resume the recognizable process of life. Do they live meanwhile ?

Science has passed through a phase in which there was a sharp distinction between “ organic’ matter, which was either in or had been produced by a living thing, and inorganic matter. While living things possessed individuality, inorganic matter was supposed to consist of repetitions of identically similar atoms and molecules. Certain modern writers, however, writing from the standpoint of mathematical physics, display a disposition to recognize individuality even in atoms, to speak of each atom as an “ organism,” ‘simpler in nature, but otherwise not dissimilar to the intricate complexes which make up definitely living tissue. This is certainly pulling the word “ organism ” down to the level of a much simpler structure than has been customary in its use. These writers believe that a process of complication is all that separates the non-living from the living, and in spite of the want of any direct evidence on the biologist’s side, they assert the credibility of spontaneous generation from their point of view. The complicated carbon compounds that are found in living things were at one time thought only to arise under the influence of a “ vital force,” and their distinctiveness was one of the most

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