834 THE POPULAR SCIENCE MONTHLY. 



to move than an ordinary bowlder, but it may be made to move with 

 a touch. So it probably is with organisms. But, if so, then the 

 causes of variation are external, as in many cases we actually know 

 them to be ; and they must depend on instability or change in sur- 

 roundings, and this so arranged as not to be too extreme in amount, 

 and to operate in some determinate direction. Observe how remark- 

 able the unity of the adjustments involved in such a supposition. 

 How superior they must be to our rude and always more or kss unsuc- 

 cessful attempts to produce and carry forward varieties and races in 

 definite directions ! This can not be chance. If it exists, it must 

 depend on plans deeply laid in the nature of things, else it would be 

 most monstrous magic and causeless miracle. Still more certain is this 

 conclusion when we consider the vast and orderly succession made 

 known to us by geology, and which must have been regulated by 

 fixed laws, only a few of which are as yet known to us. 



Beyond these general considerations, we have others of a more 

 special character, based on paleontological facts, which show how im- 

 perfect are our attempts, as yet, to reach the true causes of the intro- 

 duction of genera and species. 



One is the remarkable fixity of the leading types of living beings 

 in geological time. If, instead of framing, like Haeckel, fanciful phy- 

 logenies, we take the trouble, with Barrande and Gaudry, to trace the 

 forms of life through the period of their existence, each along its own 

 line, we shall be greatly struck with this, and especially with the contin- 

 uous existence of many low types of life through vicissitudes of physi- 

 cal conditions of the most stupendous character, and over a lapse of 

 time scarcely conceivable. What is still more remarkable is, that this 

 holds in groups which, within certain limits, are perhaps the most 

 variable of all. In the present world no creatures are individually 

 more variable than the protozoa ; as, for example, the foraminifera 

 and the sponges. Yet these groups are fundamentally the same, from 

 the beginning of the palaeozoic until now ; and modern species seem 

 scarcely at all to differ from specimens procured from rocks at least 

 half-way back to the beginning of our geological record. If we sup- 

 pose that the present sponges and foraminifera are the descendants 

 of those of the Silurian period, we can affirm that, in all that vast 

 lapse of time, they have, on the whole, made little greater change than 

 that which may be observed in variable forms at present. The same 

 remark applies to other low animal forms. In forms somewhat higher 

 and less variable, this is equally noteworthy. The pattern of the vena- 

 tion of the wings of cockroaches and the structure and form of land- 

 snails, gaily- worms, and decapod crustaceans were all settled in the 

 carboniferous age in a way that still remains. So were the foliage 

 and the fructification of club-mosses and ferns. If at any time mem- 

 bers of these groups branched off, so as to lay the foundation of new 

 species, this must have been a very rare and exceptional occurrence, 



