968 LIFE : OUTLINES OF GENERAL BIOLOGY 



rate flowering; so here may well be a stimulus of variation, and that 

 cumulative, towards the more vegetative or more floral types so 

 frequent in many species and even in allied genera. 



ILLUSTRATION OF A METHOD OF ORGANIC EVOLUTION 



CHANGE OF FUNCTION.— Part of what may be called the 

 method of organic evolution is the differentiation of some new 

 structure out of an older and more generalised structure, of different 

 fimction. This may come about in various ways; a secondary 

 function of the old structure may become the dominant function, 

 as when a swim-bladder becomes a lung, or vice versa ; or a struc- 

 ture may change in the time of its development, as when the 

 amphibian cloacal bladder becomes a reptilian foetal membrane ; or 

 when one of a set of similar appendages, so abundantly represented 

 that a pair can be dispensed with, varies in a manner that makes a 

 new use possible, as when a crustacean swimmeret becomes a male 

 reproductive appendage. There are other possible ways in which a 

 change of function may be interpreted, but it will be profitable to 

 take the facts first, and their theory later. 



(i) The Eustachian tube, present from Amphibians to Mammals, 

 is a continuation of the outer ear passage to the back of the mouth. 

 In development it arises from the visceral cleft that lies between the 

 first two visceral arches, the mandibular and the hyoid. The distal 

 part of the cleft dilates to form the tympanic cavity, across which 

 the drum of the ear is stretched, and the proximal part becomes the 

 Eustachian tube leading into the pharynx. There can be no doubt 

 that the Eustachian tube, which has no connection with respiration, 

 answers to part of the hyomandibular gill-cleft in fishes — the one 

 which is known as the spiracle in skate and shark, and allows of 

 the entrance of the water used in breathing. It includes in cartila- 

 ginous fishes, such as the two mentioned, a minute vestigial gill. 

 With the transition from water to dry land, and the associated 

 replacement of gills by lungs, the gill-clefts lost their primary 

 function. In Reptiles, Birds, and Mammals they practically disappear 

 except as embryonic vestiges; but the first one persists as the 

 Eustachian tube. That is to say, a respiratory cleft, the first of a 

 dwindling series, finds a new utility, so persists and develops accord- 

 ingly. The five points are : {a) the disappearance of an old function ; 

 (b) the dwindling of the structures concerned; (c) the appearance of 

 a new function associated with new organs (lungs) ; {d) the ancillary 

 utilisation of one of the old dwindling structures, persisting by 

 organic momentum in embryonic life, and {e) its persistence in the 

 adult. 



(2) Somewhat simpler is the viper's poison-apparatus. The 



