The last thirty years have yielded much palæontological evidence of the successive stages of species-transformation. In quietly deposited horizontal strata of the earth's crust, lying one above another, the whole phyletic history of a group of snail-species has repeatedly been found in historic order, the oldest in the deepest layer, the youngest in the uppermost, and the numerous and often very divergent 'species' of a particular deposit are connected by transition forms in the intermediate strata. From the point of view of time, therefore, these are not 'typical' species, but circles of forms in a state of variability.
The most beautiful of such cases are the Planorbis species from the small lacustrine deposits of Steinheim in Swabia, the Paludina strata of Slavonia, and various groups of Ammonites.
These cases have been described and discussed so often that I need only refer to their most essential features.
The Planorbis strata of Steinheim were first investigated, from the point of view of the theory of descent, by Hilgendorf (1866). He described nineteen different varieties, which, as they are all connected in chronological succession with each other, he grouped together under the name of Planorbis multiformis. These little freshwater snails are found in millions in the strata of the former lake-basin of Steinheim, and they are arranged in so orderly and regular a manner that two observers, working independently and at different times, succeeded in building up the genealogical tree in almost the same way. According to Alpheus Hyatt, the later investigator, all the forms are derived from one ancestral form, Planorbis lævis, from which four different series have descended, one of them splitting up again into three subordinate series.
All the individual members of these series are connected by intermediate forms in such a manner that a long period of constancy of forms seems to be succeeded by a shorter period of transformation, from which again a relatively constant form arises.
We see, therefore, that the idea of species is fully justified in a certain sense; we find indeed at certain times a breaking up of the fixed specific type, the species becomes variable, but soon the medley of forms clears up again and a new constant form arises—a new species, which remains the same for a long series of generations, until ultimately it too begins to waver, and is transformed once more. But if we were to place side by side the cross-sections of this genealogical tree at different levels, we should only see several well-defined species between which no intermediate forms could be recognized; these would only be found in the intermediate strata.
The problem we have now to discuss is, how it comes about that relatively sharply defined species exist which are connected with ancestral forms further back, but which form among themselves an exclusive, more or less homogeneous, host of individuals. How does it happen that we everywhere find a specific type, and not an endless number of individual forms connected with one another in all directions?
This would require no further explanation if a phyletic evolutionary force impelled the forms of life to vary in a definite manner, and thus to become transmuted into new forms in the course of generations. In that case the whole genealogical tree of the organisms on the earth must have been potentially contained in the lowest moneron, so that, given time and the most indispensable general conditions of existence, the living world just as we know it must have resulted. Nägeli was the first to express this view, and he followed it out consistently, not even hesitating to deny the existence of all processes of selection, and to represent the whole of evolution as a process conditioned by this phyletic force, which would have given rise to the world of organisms which has actually arisen, even if the conditions of life at the different periods of the earth's history had been other than they were. I have always combated this idea, without however overlooking that it is based upon facts which—at that time at any rate—gave it a certain justification. We cannot pass it by without giving some other interpretation of the facts. Following Nägeli, the botanist Askenasy championed this view of 'variation in a different direction,' which gives rise to new forms; and in more recent times Romanes, Henslow, and Eimer expressed similar views, and—although they did not actually dispute the existence of processes of selection—they attributed a much less important rôle to them, and referred the phyletic genealogical tree of organisms in the main to other and internal causes.
Like Nägeli himself, his followers have laid stress upon the fact that natural selection cannot be the cause of the evolution and succession of particular species, because the differences which separate species from species are not of an adaptive nature, and therefore cannot depend upon selection; but if the step from one species to the next succeeding one does not depend upon adaptation, then the greater steps to genera, families, and orders cannot be referred to it either, since these can only be thought of as depending upon a long-continued splitting up of species. Genera, families, and all higher groups must be recognized as conventional categories, not as real divisions existing in nature itself. Even Treviranus and Lamarck maintained that the differences between genera depended just as much upon our estimate, our intellectual convenience, as do the differences between species. All forms were originally connected, though they may not be so now, and if the species are really not distinguished by adaptive characters, then neither are any other grades of our classificatory system, neither order nor classes, since they all depend originally on the transmutation of species. It was therefore quite consistent of Nägeli to seek the mainspring of organic evolution, not in adaptation, but in an unknown evolutionary force. Thus he refused to recognize adaptation as a consequence of selection, but regarded it, as Lamarck had done, as the direct effect of external conditions, and as an entirely subordinate factor in the transmutation of forms.