STRUCTURAL MODIFICATIONS 133 



and obliges the organs to change their function. These changes 

 of function are frequent in the animal kingdom. Anton Dohrn, 

 the founder of the famous Aquarium at Naples, called attention 

 to these in an admirable series of memoirs. 1 Flying fishes make 

 use in flight of anterior fins developed originally for 

 swimming ; the abdominal and anal fins of Gobins, Liparis 

 and Lepadogaster are transformed into suckers to facilitate 

 fixation ; the anterior part of the dorsal fin becomes a fixed 

 cephalic sucker among the Echeneididae ; Savigny has shown 

 how largely the buccal appendages of Insects can be modified, 

 according to the very varied food of these creatures ; the anal 

 terebra, serving as the ovipositor of Hymenoptera with 

 phytophagous or entomophagous larvae, becomes the defensive 

 sting of Bees, Wasps, and Ants ; the mouth of the Vertebrates 

 is a former branchial slit, etc. We might almost say that all 

 comparative anatomy is but an account of similar changes 

 of function — the very opposite of pre-adaptations. No more 

 than pre-adaptations can these explain everything, and with 

 them they merely furnish a basis, still too narrow, for a complete 

 theory of organic transformations. But if we bear these facts in 

 mind we shall be better able to recognize the determining causes 

 of those persistent characters which are found in all animals 

 of the same group, and lend to each group a special 

 physiognomy. To go back to their original cause, it will suffice 

 to call to our aid the fundamental principles of embryogeny 

 described in an earlier chapter. Let us first see how the 

 Echinoderm type was arrived at, whose larva} are free, or are 

 only fixed at a late stage, and which yet produce organisms 

 definitely radiate, i.e. ramifie, a phenomenon that at first 

 sight appears contrary to the laws that have determined the 

 two main types of animal structure. 



Dominating the almost infinite variety that armogenesis 

 and tachygenesis together have imposed upon the embryonic 

 forms of Star-fishes, Sea-urchins, Holothurians, and Crinoids 

 forming the phylum of the Echinodermata, certain constant 

 characters appear that are essentially patrogonic, that is to 

 sa}', representing phases of the phylogenetic evolution of the 

 ancestors of the present Echinoderms. Whatever be the 

 external form taken by these embryos, they first present at 

 their birth a distinct bilateral symmetry. Their dorsal convex 



1 LXVIII. 



