From histology has sprung the science of cytology (q.v.), which is devoted to the discovery and elucidation of the visible structures within cells themselves. These researches have proved to be of such great significance for zoology in general that they will be referred to in later portions of this article.
It very soon became clear, largely, in all probability, through the investigations of the anatomy of man, that, within certain limits, the structure of all individuals of any particular kind of animal was the same. It then became possible to compare the structures of different animals with one another, and thus to gain an understanding of the nature of their differences.
Comparative anatomy has gradually led zoologists to the con clusion that the variations in structure which occur amongst animals are not haphazard in their nature, but that they are systematic, so that every animal conforms to one of a very limited number of fundamental plans of structure. There is, for example, a single pattern underlying the structure of two animals differing superficially so greatly as a fish and a bird, or a sea cucumber and a starfish. On this fundamental basis all kinds of variations may be imposed, so that the animals which exhibit them may-be fitted for life under most varied conditions.
Thus it is possible to draw up a classification of the animal kingdom based entirely on resemblances of structure. But any such process involves a deliberate choice from the great number of separate characters visible in an animal of certain structures as those on which weight shall be placed in judging its position in the classification. This process involves extended comparisons between widely differing animals, structures which are widely dis tributed through the animal kingdom being used for the discrimi nation of the more fundamental groups, whilst those which are less widely spread serve for the establishment of smaller divisions. Though very simple in principle, this process of zoological classi fication is difficult in practice, requiring much judgment in those who are devoted to it.
The difficulties are of many different kinds; they may arise from the necessity of drawing arbitrary lines of division in nearly con tinuous series of forms, or they may depend on the existence of gaps in the series so large that the structures of the animals that they separate may differ so greatly that there is a real difficulty in discovering whether or not a comparison is possible.
It was to meet difficulties of the latter type that the science of morphology (q.v.) came into existence.
The classifications of the animal kingdom drawn up by zoolo gists before 1859 endeavoured to give a strictly objective repre sentation of the facts of animal structure and of the structural relationships of animals to one another. The resulting series of groups, each included in others of higher order, was devoid of all symmetry and of all evidence of plan. It was unintelligible to men who believed that the whole system, in all its detail and com plexity, had arisen at the fiat of a Creator. But the real nature of this classification at once became clear with the resuscitation by Darwin of the theory of evolution (q.v.) and its universal accep tance. The natural classification of animals, though founded on the resemblances in structure, is really of the nature of a genealogi cal tree, expressing the actual blood relationships between animals, a tree imperfect, not only from failure of analysis of the available evidence, but also from the gaps and breaks which necessarily exist in it from the fragmentary nature of our knowledge of extinct animals.
Thus a classification drawn up on evolutionary lines necessarily takes account of time; it cannot be expressed by a linear arrange ment, or even on a surface, but involves at least a third dimension.
This tree, representing as it does the branching out of the animal kingdom during the long course of evolution, itself depen dent not only on the intrinsic properties of living matter but also on the local peculiarities of the inorganic environment, is neces sarily devoid of obvious symmetry; it includes branches tenta tively put forth which, after a longer or shorter period, proved unsuccessful, as well as those which survive to-day.