It is certainly not the least interest in studying these early forms that one is able to trace the analogy they bear with the higher forms. No rigid line can be drawn between the successive stages of growth. And it should be borne in mind that, simple as is the life-process in these single-celled organisms, many of them are highly differentiated and show great complexity of structure within the narrow limits of their size. Thus among the _protozoa_, the basis of all animal life, we find very definite and interesting modes of behaviour, such as seeking light and avoiding it, swimming in a spiral, approaching certain substances and retreating from others; the organisms often, indeed, trying one behaviour after another.[10] If we realise this it becomes easier to understand how the higher types of life have developed from these primitive types. Indeed, all the bodies of the most complex animals--including ourselves--originate as simple cells, and in the individual history of each of us divide and multiply just as do the cells which exist independently; only in multicellular organisms each cell must be regarded as an individual, modified to serve a special purpose, one cell differentiated to start a lineage of nerve cells, another a lineage of digestive cells, yet another for the reproduction of the species, and so on, each group of cells taking on its special use, but the power of division remaining with the modified cell. Thus a new life is built up--a child becomes an adult, by multiplication of these differentiated cells, repeating the original single-cell development. Budding, the second, and perhaps the most usual mode of asexual propagation, may be said to mark a further step in the development of the reproductive process. Here the mother-cell, instead of dividing into two equal parts and at once rupturing, protrudes a small portion of its substance, which is separated by a constriction that grows deeper and deeper until the bulk becomes wholly detached. This small bud then grows until it attains the size of the parent, when it, in turn, repeats the same process. This mode of reproduction is common to the great majority of plants. In animal life it is not confined to single-celled organism, but takes place in certain multicellulars, such as worms, bryozoans, and ascidians; one very interesting example being the sea-worm (_myrianida_) which buds off a whole chain of individuals. Nearly allied with budding is the third stage, in which the division is multiple and rapid within the limited space of the mother-cell. This is known as spore formation. The cells become detached, and do not further develop until they have escaped from the parent. They then increase by division and growth to form independent individuals. This spore reproduction is found among certain types of vegetation; it also occurs in the _protozoa_.