RUDIMENTARY ORGANS. 31 



and sessile sea-squirts at first free-swimming, always undergo 

 degeneration. The retrogression can be seen in each life- 

 time. But the little Kiwi of New Zealand, with mere 

 apologies for wings, and many cave-fishes and cave- 

 crustaceans with slight hints of eyes, illustrate degeneration 

 which has taken such a hold of the animals that the young 

 stages also are degenerate. The retrogression cannot be 

 seen in each lifetime, evident as it is when we compare 

 these degenerate forms with their ancestral ideal, {c) But 

 by " rudimentary organs " we mean very often structures 

 somewhat different, e.g., the branchial or visceral clefts 

 which persist in embryonic Reptiles, Birds, and Mammals, 

 though they serve no obvious purpose, or the embryonic 

 teeth of whalebone whales, of the duckmole, and of parrots. 

 These are " vestigial structures," traces of ancestral history, 

 and explicable on no other theory. The branchial cleits 

 are used for respiration in all Vertebrates below Reptiles : the 

 ancestors of whalebone whales, of the duckmole, of Birds, 

 doubtless had functional teeth. In regard to these persistent 

 vestigial structures, it must also be recognised that we are not 

 warranted in calling them useless. Though they themselves 

 serve no obvious purpose, they may be, as Kleinenberg sug- 

 gests, the necessary conditions of some useful structure. 



Classification of Organs. — We may arrange the various 

 parts of the body physiologically, according to their share in 

 the life. Thus, some parts have most to do with the external 

 relations of the animals ; such are the external appendages, 

 locomotor, prehensile, food-receiving, protective, aggressive, 

 and copulatory. Of internal parts, the skeletal structures are 

 passive; the nervous, muscular, and glandular parts are active. 

 The teproductive organs are distinct from all the rest. They 

 are often called " gonads," and should never be called glands, 

 i^-^ Another important classification of organs is embryological, 

 i.e., aocording to the embryonic layer from which the various 

 parts ariser~ Thus, the outer layer of the embryo (the 

 ectoderm or epiblast) forms in the adult (i) the outer .skin 

 or epidermis, (2) the nervous system, (3) much at least of 

 the sense-organs : the inner layer of the embryo (the 

 endoderm OT, hypoblast) forms at least an important part 

 (the "mid-gut") of the food-canal, and the basis of outgrowths 

 (lungs, liver, pancreas, etc.) which may arise therefrom, and 



