368 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 difter- 

 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. 



Which, then, is the most rational hypothesis ? — that of 

 special creations w^hich 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 who are not familiar 

 with zoology, and who have not seen how clear becomes 

 the relationship between the simj)lest 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 forms, by insensible gradations. 



