320 SCIENCE PROGRESS 



takes place. The process of Evolution depends on the fact that similar 

 forces (of the environment), acting on similar structures and forces (of the 

 germs that grow into parent and offspring), produce similar results (in the 

 fully developed animals, parent and offspring alike) ; while their dissimilarities 

 are caused by the spontaneous variation of the germ, by the inequalities of 

 the environment, and by constantly repeated precocity, which brings the 

 offspring to the point reached by the parent while it has yet power to respond 

 by further modification to the incident forces of the environment. 



Whatever therefore may be the actual changes occurring in the germ, 

 none is required by my theory except such as will cause a repeated accelera- 

 tion of its development. I need hardly point out how well this agrees with 

 recent discoveries of the function of more than one of the ductless glands to 

 elaborate and discharge into the blood certain " Hormones," or chemical 

 stimulants of growth. 



Now from contemplation of the series of individuals in a line of descent, 

 let us turn to the characters, or groups of characters, to which their efficiency 

 may be due. What shall we see in these ? Evidently, if they are reached 

 by each generation in turn at a slightly earlier period of growth, all the 

 successive phases of the ontogeny must be as it were in a state of constant 

 movement from the end towards the beginning of the series. It is this 

 steady backward march of all characters of all creatures, but especially of 

 the higher, on which I wish to focus attention, and to which in the above- 

 mentioned Article I gave the name of Precession, owing to the clear though 

 distant analogy which it bears to the Precession of the Equinoxes in the solar 

 system. The choice of a name no doubt matters not much, but I think it 

 highly desirable that this hitherto unnamed universal movement should 

 have one, as being a phenomenon of the highest importance in the organic 

 world. 



Besides precocity, it will readily be seen that it is this movement which 

 has brought about what is called " Recapitulation " in embryonic growth — 

 the well-known fact that every animal in its early growth passes through 

 phases that indicate in some degree, be it great or small, the remoter ancestry 

 from which its species has sprung. The Ontogeny, in technical language, 

 recapitulates in a few months at the longest, the Phylogeny, or secular evolu- 

 tion of the race. The backward movement of all characters, coupled with 

 the constant reduction of their growth-time, leads eventually to the more 

 ancient of them being crowded together into the earlier periods of growth, 

 and recapitulated instead of undergoing the almost exact repetition shown 

 in offspring of all the successive developments of the parents. What were 

 once the final and crucial attainments of growth, and determined the pre- 

 ferential survival of their possessors as the select out of the common herd, 

 pass back by Precession until they become inherent growth-properties of the 

 germ, the common birth-right of every member of the species ; and then, 

 having survived their usefulness, still pass back so as to become temporary 

 and unimportant phases of embryonic growth, like the external tail in man, 

 still conspicuous in his embryo ; and then are yet further reduced, until, 

 like the hind leg-bones in Sperm-whales, of which it is said no rudiment now 

 appears even in embryonic growth, they finally pass out of existence altogether. 



By way of exemplification let us consider a little the question of the 

 teeth, which owing to their hardness have survived in such numbers in a 

 fossil state as memorials of the fauna of ancient times. From these it is 

 clear that in many cases a dentition that was a useful and important character 

 of the maturity of an ancient creature now known only by its fossilised remains, 

 appears in its modern descendants merely as an evanescent development of 

 immaturity, rapidly formed perhaps long before birth, only to be as quickly 

 reabsorbed. Or possibly it never even appears at all. Thus at one time 

 all birds were toothed, including the ancestors of all modern species, while 



