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 series, 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 series of metamorphoses equally great 

 with each of those which the h3^othesis 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. 



