THE SIGNIFICANCE OF A NATURAL SYSTEM OF CLASSIFICATION. 463 



they happened to be located near the margin of the rapidly infolding neural plate. 

 One may observe in the arthropod stock the steady approach of this inevitable 

 disaster to the visual organs, brought about by the increasing precocity of the 

 embryonic brain and optic ganglia. It is important to observe that under the 

 existing conditions there is no half-way position for the eyes; they are either carried 

 wholly inside, or remain wholly outside the brain, and that once inside there is no 

 escape for them. But the very slight difference in the physical conditions that 

 finally precipitates the eyes into the brain cavity, creates at once an immense 

 difference in the physical conditions under which the eyes, the brain, and indeed 

 the whole anterior part of the head must complete its development. At one stroke 

 the lateral eye is changed from the superficial type seen in the invertebrate to that 

 which, in the vertebrate, lies in the walls of the cerebral vesicle. 



The actual closing of the old mouth and the opening of the new one, the 

 transfer of the oral arches to the haemal surface of the head, and the transfer of 

 the lateral eyes to the interior of the cerebral vesicle, brought about a great crisis 

 in the evolution of the arthropod-vertebrate stock; and the successful consum- 

 mation of these internal organic changes constitutes the most important event in 

 the evolution of the animal kingdom. 



The organic adjustments referred to above were necessarily rapid in their 

 progress and revolutionary in their effect. But the evolution of the conditions 

 that led up to them was extremely slow, consisting of a long series of cumulative 

 internal events that had no immediate bearing on the use or the character of the 

 organs that in the end were most vitally affected. In the arthropods, we may 

 follow in detail through an immensely long period of time, and in a long series 

 of animals, the steps that led up to this inevitable crisis. The rapid succession 

 of readjustments that followed gave rise to a sharply defined, remarkably short- 

 lived, transitional phylum, the ostracoderms. After that stage is passed, the 

 organs in question remain practically stationary through the whole vertebrate 

 series from fishes to man. 



THE SIGNIFICANCE OF A NATURAL SYSTEM OF CLASSIFICATION. 



A consideration of the facts discussed above, to which many more might 

 be added, throws a new light on variation and environment and on the meaning 

 of a natural system of classification. 



A natural system of classification is an attempt to represent in graphic 

 form a genealogical tree of the animal kingdom, and in so far as it is a true record 

 of evolution and descent, it should reflect the guiding and controlling factors that 

 have created it. 



If natural selection and external environment are the important factors 

 in creative evolution that they are frequently assumed to be, then there should 

 be reflected in a natural system of classification a broad correlation between 

 structure and environment that could be used as an aid in the making of it. But 

 this is not the case, for we do not usually divide animals, according to their mode 



