EVOLUTION AND MANKIND. 289 



the past. Even at the present time, some animals retain struc- 

 tures df apparently little or no use to them, in the form of 

 vestigial organs. The function of such organs is gathered from 

 considerations of other animals in which they are of use. For 

 example, movability of the external ears by the action of 

 powerful ear muscles is common among the hoofed animals. In 

 man, the ear muscles are vestigial, btit it is very probable that 

 they were both larger and more ftinctional in ancestral man. 

 The gill slits oif developing reptiles, birds and mammals are also 

 vestigial, except in the case of the first gill slit, which survives 

 as a connection betw^een the ear and the throat, and is known 

 as the Eustachian tube. Other examples of vestigial organs are 

 the minute " third eyelid " of man, lying in the median angle of 

 the eye, the rudimentary wing of birds like the ostrich and the 

 New Zealand kiwi or Apteryx, the hind limbs of the whale sunk 

 beneath the skin, and the teeth of the baleen whale that never cut 

 the gum. Also, there is good reason to believe that remote 

 fossil ancestors of existing vertebrates possessed an additional 

 pair of eyes on the top of the head. These extra eyes were 

 known as pineal or parietal eyes, and traces of them are still to 

 be found in lampreys and Hzards, and especially in the remark- 

 able New Zealand reptile, Sphenodon punctatus. This lizard- 

 like animal is apparently the oldest surviving terrestrial verte- 

 brate. Its pineal eye possesses retina, lens and nerve, but lies 

 buried beneath the skin, though it pierces the bones of the skull. 

 It is the left-hand member of the original pair of eyes that per- 

 sists in Sphenodon. 



The embrvological evidences of evolution are complicated 

 by specialisation, but are none the less clear. Different animals 

 pass through similar stages in development, and animals that are 

 more alike resemble one another longer during development. 

 Thus, the egg or ovum of all animals is at first a single cell, 

 comparable with the single-celled animals known as Protozoa, of 

 which an Amoeba such as Entamoeba histolytica, the excitant of 

 amoebic dysentery, is an example. Later, in all animals higher 

 in development than the Protozoa, a two-layered embryo is pro- 

 duced, whether the organism be a jellyfish or a mammal. The 

 higher invertebrates and all vertebrates develop three germinal 

 layers, as may be seen from a consideration of the development 

 of Amphioxus. All vertebrates at a still later stage of develop- 

 ment have gill slits, and possess a notochord which is the fore- 

 runner of the backbone. The simplest explanation of these 

 embryological facts is that in development the animal broadly 

 recapitulates its ancestry. This generalisation was first enunciated 

 by von Baer early in the 19th century. For ex:ample, the frog's 

 tadpole recapitulates a fishlike ancestral stage. Regarding this 

 recapittilation hypothesis, which Haeckel called the biogenetic 

 law, it has been well said, " The living hand of the past is upon 

 the embryo, constraining it to follow the old route of its race, 

 and often reasserting its power in trivial details, even when a 

 considerable short cut has been made." However, it must be 



