10 
4. The highest forms of one epoch have not had their origin 
in the highest forms of the preceding epoch, nor have they given rise 
directly to the highest forms of the succeeding one. 
Nothing, then, could be further from the truth than that the 
dominant species that characterise successive epochs of the 
world’s history are linked to one another in a continuous 
uninterrupted series. Nature knows nothing of such a serial 
progression. It exists only in idea. There is no linear arrange- 
ment of higher or lower forms. All forms are connected, un- 
doubtedly, but if it be possible to represent or picture to ourselves 
the method and manner in which they are linked together z” space 
of two dimensions, it must be under the figure of a tree. 
Now this picture of a growing tree implies a principle and 
method of such great importance in the evolution of life in all its 
forms, and in all its manifestations, that it will be advisable per- 
haps to adduce some illustrations of it. It is useful in many 
respects ; but there is danger that, if its analogies are pressed 
too far, we may be led into error. 
The bird we may say is higher and comes later in time than 
the reptile. But if we may follow back their ancestral genealogies 
they will gradually approach one another as we recede, until we 
come to a point where the branches unite nearer the trunk, and 
bird and reptile are one. The lowest reptile and the lowest bird 
are nearly allied. 
Animals are divided, as you know, into two great kingdoms of 
Vertebrates and Invertebrates. But the highest Invertebrate 
has not given rise to the lowest Vertebrate. In going down the 
main branch of the Vertebrates we pass by as it were the highest 
point of the branch representing the Mollusc, and find a relation- 
ship between them far indeed towards the base of that tree from 
which they both seem to spring. 
It is one of the most interesting of researches, this of the 
genealogy of life. But the record is often broken, and we seek in 
vain for some evidence of the earlier course which a genus or 
species has followed. Paleontology, like human history, has its 
“dark ages.” The records of the past bear witness to the deep 
pulsations of the great heart of the world, to the ebb and flow of 
the tides of the great ocean of life. There are periods when the 
creative power is comparatively dormant, and again those when 
its energies gradually rise to their full scope, and attain their 
highest point. It is as the oscillation of the pendulum, the swing 
of the earth in its orbit, or the alternations of sunlight and 
shadow on its surface. The morning and evening of the world’s 
first day, symbols of activity and rest, are repeated in larger periods 
of rhythmic change throughout the countless millenniums of its 
past. 
