Roi/al Microscopical Socidy. 1 09 



Macroura, almost every species difiers from every other in the form 

 of the zoosperms, yet they may be all referred to the same type. 

 In the Isopoda and lower crustaceans, forms which as yet I have 

 not much examined, the zoosperms are linear, resembling those of 

 other classes in the animal kingdom. Now, if the facts of nature 

 mean anything at all, all crustaceans are derived from one primitive 

 form, as Fritz Miiller * has shown to be extremely probable ; and it 

 becomes interesting to inquire how, as the external forms of the 

 crustaceans became modified, the zoosperms became modified also, 

 for it is evident that these changes of the external form were 

 simply the indications of more profound variations going on within. 

 Natural selection does not appear to me to be competent by itself 

 to effect this, neither do I believe in the single law which is sup- 

 posed by some authors to impel species to vary in some definite 

 direction ; rather, I think, it must be due to a collocation (to use a 

 phrase rendered classical by the late Mr. John Stuart Mill) of most 

 intricate laws acting and reacting on each other, and which, in the 

 present state of our faculties, it seems impossible to analyze, but 

 which might perhaps be made more plain by an illustration. A 

 cod-fish, for instance, as is well known, lays some thousands of eggs ; 

 of these eggs some, say a dozen, grow up to maturity : none of 

 these thousands of eggs and young fish perish by chance, but their 

 mode and time of dying depend on an immense number of circum- 

 stances, such as hereditary disposition, greater or less abundance of 

 enemies, heat, cold, or density of their surrounding medium, and 

 thousands of other factors of which we know nothing. In some 

 such manner as this do the laws of nature combine to alter the 

 constitution of animals and form new species. Natural selection 

 itself is but a metaphorical term applied to a certain fraction of 

 these laws. 



In addition to the zoosperms of Crustacea I have examined those 

 of a few other invertebrata ; among them were specimens of the 

 Epeira diadema. In these spiders the zoosperms (Fig. 9) are re- 

 duced to great simplicity ; they consist of a round body, which 

 possesses no appendages whatever in the way of rays or tail. 

 When first placed on the slide these bodies appear quite homo- 

 geneous, but after a short time a circumferential portion separates 

 in appearance from an internal portion ; on the addition of water, 

 or even after they have remained some time on the slide, they 

 burst, the internal portion disappears, the external portion remains 

 as a spiral or curved thread, somewhat resembling a very short 

 linear zoosperm ; in such a condition it appears that Leydig t saw 

 them, for he has given a figure of the zoosperms of Epeira which 

 presents a certain likeness to these filaments. That this thread is 



* Fiir Darwin, translated by W. S. Dallas, 

 t 'Lelirbuch der Histologie,' tig. 261. 



