Each successive formation has therefore presented us with animals becoming gradually more and more like those now in existence; and though there is an immense and striking difference between the Silurian animals and those of to-day, this difference is greatly reduced if we compare the Silurian fauna with the Devonian; _that_ again with the Carboniferous; and so on till we reach the present.
It follows from the above that the animals of any given formation are more like those of the next formation below, and of the next formation above, than they are to any others; and this fact of itself is an almost inexplicable one, unless we believe that the animals of any given formation are, in part at any rate, the lineal descendants of the animals of the preceding formation, and the progenitors, also in part at least, of the animals of the succeeding formation.

In fact, the palaeontologist is so commonly confronted with the phenomenon of closely-allied forms of animal life succeeding one another in point of time, that he is compelled to believe that such forms have been developed from some common ancestral type by some process of "_evolution_." On the other hand, there are many phenomena, such as the apparently sudden introduction of new forms throughout all past time, and the common occurrence of wholly isolated types, which cannot be explained in this way.

Whilst it seems certain, therefore, that many of the phenomena of the succession of animal life in past periods can only be explained by some law of evolution, it seems at the same time certain that there has always been some other deeper and higher law at work, on the nature of which it would be futile to speculate at present.
Not only do we find that the animals of each successive formation become gradually more and more like those now existing upon the globe, as we pass from the older rocks into the newer, but we also find that there has been a gradual progression and development in the _types_ of animal life which characterise the geological ages.

If we take the earliest-known and oldest examples of any given group of animals, it can sometimes be shown that these primitive forms, though in themselves highly organised, possessed certain characters such as are now only seen in the _young_ of their existing representatives.

In technical language, the early forms of life in some instances possess "_embryonic_" characters, though this does not prevent them often attaining a size much more gigantic than their nearest living relatives.