434 THE EVOLUTION OF LIFE. 



By continuing this process the ellipse changes insensibly into 

 a parabola. On still further diminishing the angle, the para 

 bola becomes an hyperbola. And finally, if the cone be 

 made gradually more obtuse, the hyperbola passes into a 

 straight line as the angle of the cone approaches 180. Here 

 then we have five different species of line circle, ellipse, 

 parabola, hyperbola, and straight line each having its pecu 

 liar properties and its separate equation, and the first and 

 last of which are quite opposite in nature, connected together 

 as members of one scries, all producible by a single process of 

 insensible modification. 



But the experiences which most clearly illustrate the pro 

 cess of general evolution, are our experiences of special 

 evolution, repeated in every plant and animal. Each organ 

 ism exhibits, within a short time, a series of changes which, 

 when supposed to occupy a period indefinitely great, and to 

 go on in various ways instead of one way, give us a tolerably 

 clear conception of organic evolution at large. In an indi 

 vidual development, we see brought into a comparatively 

 infinitesimal time, a scries of metamorphoses equally great 

 with each of those which the hypothesis of evolution assumes 

 to have taken place during immeasurable geologic epochs. A 

 tree differs from a seed in every respect in bulk, in struc 

 ture, in colour, in form, in chemical composition. Yet is the 

 one changed in the course of a few years into the other: 

 changed so gradually, that at no moment can it be said 

 Now the seed ceases to be and the tree exists. What can be 

 more widely contrasted than a newly-born child and the 

 small, semi-transparent, gelatinous spherule constituting the 

 human ovum? The infant is so complex in structure that a 

 cyclopaedia is needed to describe its constituent parts. The 

 germinal vesicle is so simple that it may be defined in a line. 

 Nevertheless, nine months suffice to develop the one out of 

 the other; and that, too, by a series of modifications so 

 small, that were the embryo examined at successive minutes, 

 even a microscope would not disclose any sensible changes. 



