The science of life : fully illustrated in tone and line and including many diagrams

ER. Gri kite.

Fig. 141.

THE SCIENCE OF LIFE

CHAPTER 4

Two freely interbreeding species of crows.

The Hooded Crow, Corvus cornix, is above on the left; the Carrion Crow, Corvus corone, is above on the right.

The other

birds are hybrids, showing the hooded pattern more or less distinctly.

them both. The early systematists blamed their own incompetence. They believed in species, even where they could not recognize them, and they spent an enormous amount of time and energy in the quest of universal diagnostics of species (such as the test of mutual fertility that we have just discussed) and to devising definitions of species which would apply to every case. But the nineteenth century saw the passing of the idea of separately created “‘ kinds.» When the fact of Evolution was clearly stated and gained general acceptance, the confusion and dif_iculty of systematic work suddenly became luminously intelligible. It is precisely what one would expect. It is in itself evidence that Evolution is taking place. Our ideas of species have undergone a change and it is important to realize just how profound that change has been.

We have already spoken of life, seen as a whole in time, as a tree ;_ the vertical height of the tree represents the time-dimension, and its branches, forking and multiplying from a common trunk, represent the various lines along which living things have evolved, diverging and spreading away from some common ancestral form. The horizontal distance between any two twigs represents

254:

the difference between two races, the degree to which they have diverged from their common stem. Clearly, the spectacle which life presents at any particular time—the present, for example—will be represented by a horizontal slice through the tree at an appropriate level. Now, what would such a slice reveal? It would pass through a great number of separate twigs; they would appear on the section as circles, and in our analogy they represent those species that are sharply marked off and distinct—such for example as the so-called Maned Wolf of South America. Some would be close together—if they had recently branched from a common stem—and others would be far apart; the former are closely related, the latter distantly related species. But here and there our section would pass through an actual fork—it would appear as an ellipse, or a figure of eight, or two circles just touching each other—and these correspond to the doubtful cases, the border-line cases, like the case of the tigers, that may with equal justice be regarded as several species or as one. In a word, the twigs in our section would show every sort of relation to each other, every conceivable grade from twigs that are just branching to twigs that lie far apart. And