[99] THE EVOLUTION OF THE FINS OF FISHES. 1079 



The directions of the axes of the cleavage spindles change with each 

 cleavage and stand at nearly right angles to each other. The directions 

 of the spindles of alternate generations of cells are therefore for some- 

 time nearly coincident in direction. 



These phenomena, when contemplated in reference to the origin of 

 variations, are very significant ; extrinsic forces which are ever active 

 even during the earliest stages of the nascent embryo, it must occur 

 to every one, might very readily produce an impression upon a mechan- 

 ism so delicate as the one just described, where the parts can be meas- 

 ured only in units of length of infinitesimally small dimensions, or in 

 terms of thousandths of a millimeter. Variations in the action of in- 

 trinsic forces during early development need therefore be exceedingly 

 slight in order to initiate and be productive of morphological changes 

 which would cause the adult to vary within certain limits moie or less 

 from the parent in form, because these caryokinetic processes or nuclear 

 metamorphoses preside over the genesis of the tissues, and the order of 

 the manifestation of these nuclear changes seems i)redetermiued, in con- 

 sequence of which the germ of an organism may in the course of de- 

 velopment give rise to something very nearly like the parent. 



Variations in temperature affect the rate of these rhythmical meta- 

 morphoses of nuclei ; low temperatures generally retard them, while 

 higher temperatures accelerate them. This correlation between the 

 temperature of the medium in which development goes on and the vital 

 energy of the growing ovum exhibited during caryokinetic action is 

 very remarkable, as shown in the course of experiments by the writer 

 on the development of the eggs of fishes.* 



We are thus brought face to face with some of the forces which must 

 initiate variations even before there are organs present which can be 

 exercised as in the adult, but when the germ is nevertheless complex 

 enough, as the play of internal forces sufficiently shows through the ac- 

 tivity which they exhibit. Matter and motion are the principal factors 

 involved ; but the matter may be said to be so compounded and in such 

 relation to the cosmos as to be recognized as alive and capable of 

 manifesting reflex actions. That last capacity is the thing needed by 

 the doctrine of evolution as the justification of Lamarck's second law, 

 which may be so extended as to involve the consideration of the pri- 

 mary causes leading to variation of a germ in utero or in the ovary ; a 

 seed in the form of the ovicell or in the ovary of the plant. In this way 

 only does it seem conceivable that the origin of early or embryonic vari- 

 ations can be understood. 



Lamarck's third law of metamorphosis is stated thus : "The develop- 

 ment of the organs and their strength of action are constantly in pro- 

 portion to the extent or degree to which they are used." This is now 

 a recognized axiom of the doctrine of evolution. 



*Bull.U. S.Fisli Commission, I. 1881, pp. 187-190 caud 3:55-339, PI. XVIII. 



