3G8 THE DEVELOPMENT HYPOTHESIS. 



rate species. They can show, too, that the changes daily 

 taking place in ourselves the facility that attends long 

 practice, and the loss of aptitude that begins when practice 

 ceases the strengthening of passions habitually gratified, 

 and the weakening of those habitually curbed the devel 

 opment of every faculty, bodily, moral, or intellectual, ac 

 cording to the use made of it are all explicable on this 

 same principle. And thus they can show that throughout 

 all organic nature there is at work a modifying influence 

 of the kind they assign as the cause of these specific differ 

 ences : an influence which, though slow in its action, does, 

 in time, if the circumstances demand it, produce marked 

 changes an influence which, to all appearance, would pro 

 duce in the millions of years, and under the great varieties 

 of condition which geological records imply, any amount 

 of change. 



&quot;Which, then, is the most rational hypothesis? that of 

 special creations which has neither a fact to support it nor 

 is even definitely conceivable ; or that of modification, 

 which is not only definitely conceivable, but is countenanced 

 by the habitudes of every existing organism ? 



That by any series of changes a protozoon should ever 

 become a mammal, seems to those Avho are not familiar 

 with zoology, and who have not seen how clear becomes 

 the relationship between the simplest and the most com 

 plex forms when intermediate forms are examined, a very 

 grotesque notion. Habitually looking at things rather in 

 their statical than in their dynamical aspect, they nevei 

 realize the fact that, by small increments of modification, 

 any amount of modification may in time be generated. 

 That surprise which they feel on finding one whom they 

 last saw as a boy, grown into a man, becomes incredulity 

 when the degree of change is greater. Nevertheless, abun 

 dant instances are at hand of the mode in which we may 

 pass to the most diverse form^ by insensible gradations. 



