Evidences from Classification : All the known living animals and plants have been classified into various species, genera, families, order, classes, phyla and kingdoms. The classification of a particular animal is attempted only after its extensive study. It is seen that every living being is related with other living being. The relations may be very close or may be quite apart. On their relationship, they are put under various orders, classes, phyla etc. On the superficial examination one can hardly believe that they are interrelated. But after their careful study they can be arranged in definite order, Protozoa (acellular) being at the base while Chordata at the top. No doubt, there is some sort of gap between chordates and non-chordates today but who known that this gap may be filled some day by some further discoveries. Moreover, the present-day types represent only the terminal twigs of a vast phylogenetic tree and for establishing relationship we should focus our attention on the main trunk. The animals can be classified as described above in a systematic order.

Evidences from Comparative Anatomy : In all the living animals, the basic substance of life is Protoplasm. If the species had been created separately, then there should be no relationship in the various organs and systems of animals. But on the contrary, we see that large number of animals although unlike in appearance show most of the systems and organs made on the same plan. The resemblance are very close in the members of the same group. For example �
(a) Analogy and Homology : While examining the various structures in the bodies of different animals, one may come across certain organs of same origin but of different functions. For example, the forelimbs of salamander, crocodile, bird, bat, whale, and man, all have the same origin and essential structures but different functions to perform. Such structures are called homologous. On the other hand, the wings of insect, pterodactyle, bird and bat perform the same function, though they have different origin and entirely different structures. Such structures are termed analogous.
The homologous structures give us the answer that how the two different types of animals or organs have the same origin and functions to perform; this shows that they have changed themselves according to their different needs.
 Analogous organs are those, which are functionally similar.
 Homologous organs explains divergent evolution.
 Similarity developed in distantly related groups as an adaptation to the same function is called convergent evolution.
 The wings of an insect and a bat exhibit analogy.

Structures which have no function and are very much reduced. These structures are quite developed and functional in allied animals. Such useless organs are termed as vestigeal and are of frequent occurrence. In human body alone, there are as many as ninety such organs. For example, vermiform appendix is vestigeal in man but functional in rodents, horse and other herbivorous animals
 Vermiform appendix is the Vestigeal organ in man.
 Hind limbs is a vestigeal organ of python.
 Muscles of ear pinna is vestigial organ of man.
 Wisdom teeth is vestigeal organ of man.

Evidences from Physiology : Various types of chemical tests exhibit many basic similarities in physiological and chemical properties that show a physiological relationship among animals.
Take the example of thyroxin hormones, which are similar in all metabolism. The thyroid of human beings can be very easily replaced by the thyroid of cattle without any ill-effects and so on.

Evidences from Serology : This is a method by which the reactions of blood serum is observed. From the blood are also extracted the crystals of Oxyhaemoglobin. The structure differs in different vertebrates, but in a definite order. The reaction is nearly identical in man and anthropoid monkeys, but slightly less identical with other mammals.

Evidences from Embryology : With the exception of a few, every multi-cellular animal originates from a zygote. The development from zygote to adult shows many similarities in various organisms. The development is termed as ontogeny.

Haeckel, a German biologist, gave a theory �Ontogeny repeats Phylogeny�. An individual organism during its development (ontogeny) tries to repeat the history of its race by different stages (phylogeny). The vertebrate development confirms the above statement. For example, the embryos of all vertebrates pass through a gilled stage. In fishes, the gills are present in the adult condition. In amphibians, gills are present in some forms of frog and in tadpole. While in reptiles, birds and mammals these gills are never functional but nevertheless they are always present in the embryo.

Haeckel, believed that all those stages, which have occurred during the course of evolution of a particular animal, also pass through its development.
There are objections to this theory, but is throws enough light to show the interrelationship of animals.