Thus it will be seen that the formation of an animal’s body by the aggregation of cells is a necessary and ingenious way of avoiding a difficulty.
To say, however, that an animal’s body consists of cells is to take an entirely wrong starting-point. A cell is complete in itself, and can live if properly fed, even though separated from its neighbours. Many whole animals consist of only one cell. A cell is, moreover, capable of growing and dividing, thus giving rise to two cells with two nuclei, and it is only because cells find that it pays better not to separate, but to form masses and specialize at different kinds of work, that we have large animals composed of millions of cells like ourselves.
Given a cell, it is necessary to keep that cell under favourable conditions. Otherwise the unstable protoplasm breaks up. It must have the elements necessary to keep up its cycle of changes in the proper form, which we may now call food, and many cells have to go and find this requisite. It must keep away from injurious influences, and it must race other cells for localities favourable to its growth and multiplication; in fact, the cell must work.
That a cell can, in virtue of its chemical affinities alone, move about, seeking favourable conditions, showing discrimination and doing work, seems incomprehensible. In the first place, how can it move? There is only one way: it must effect a redistribution of its substance, and contrive that those parts of the cell whose activity is applied to this end shall be so situated as to produce definite changes in its shape according to the cause which evokes them. Of the way in which different cells move we shall have a good deal more to say later.
Why protoplasm should be influenced to move still requires explanation. Yet the gap between protoplasm and other substances is really not so great, after all. Heat and magnetism cause movement in inanimate matter, and the response of protoplasm as exemplified by some of the minute unicellular animals is almost as mechanical. Some kinds which swim in water move to the positive pole of a galvanic battery, others to
