372 PRINCIPLES AND METHODS OF PALAEONTOLOGY. 



stanco, is in some morphological respects more complex than the highest 

 jiiollusk, but i)hysiologically it is less so. 



One otlier (iominonly-used phrase, expressive of the relation between 

 different kinds of living beings, reqnires explanation, as ils employment 

 in an erroneous sense has led to grave errors. There is a current im- 

 pression that the lover animnls correspond with the embryonic condi- 

 tions of the higher; that, in the course of their development, the lower 

 animal advances up to a certain point and then stops, while the higher 

 goes on. 



This notion, however, is entirely incorrect ; there is no known adult 

 animal which would be regarded by any naturalist as of the same species 

 with any early condition of another animal, if the two were submitted 

 to hiiii for comparison. In no stage of their existence would a compe- 

 tent naturalist regard embryonic reptiles, or mammals, as fishes; in no 

 stage would he take an insect for a worm, or a cnttlelish for any lower 

 mollusk. The whole of this idea, the truth of which has been assumed 

 so often in geological speculations, rests upon a misunderstanding of an 

 undoubted tact, namely, that there is a time in the development of each 

 when all members of a sub kingdom resemble one another very closely, 

 and that they remain alike for a longer or shorter i)eriod according to 

 the closeness or remoteness of their affinity. Thus there is a time when 

 the embryo of a llsh could be hardly distinguished from that of a rep- 

 tile, a bird, or a mammal. But the embryo lish sooner becomes unlike 

 a mammal than the embryo reptile or bird; and the embryo quadru- 

 pedal mammal remains longer like a human embryo than does that of a 

 lish or rei>tile. 



Thus all animals in their youngest condition have, for a longer or 

 shorter time, a similar form, from which each diverges to take its spe- 

 cial configuration; if one may so say, they travel along the same road for 

 a shorter or longer distance, and then each goes aside to its own place. 

 But this is a very different matter from any one form being an arrest 

 of development of another. Of two men traveling together along the 

 great North road, one may be going to Newcastle and the other to 

 York. But it would be a very insufficient and erroneous description of 

 the journey of the one to say that is was merely that of the other cut 

 short. 



3. The next great principle of natural history of whicli some definite 

 notion must be obtained, is the doctrine of what is called the "distribu- 

 tion" of living beings. It is a matter of familiar experience that ele- 

 l)hants, lions, and rhinoceroses are not at present indigenous in Great 

 Ijiitain; and humming birds, crocodiles, and flying fish are as strange 

 to us as are the white bear, the ermine, and the musk ox. Nevertheless, 

 the latter animals are found abundantly in more northern latitudes, 

 while the former swarm within the tropics. Were any one to visit the 

 countries in which the white bear and the crocodile respectively abound, 

 he would discover that there was a certain northern limit beyond which 

 the crocodile was never seen; ami, on the other hand, that the white 

 bear never ran ~es ^outh of a given latitude. In other words, the white 

 bear and the crocodile are found within, or are distributed over, certain 

 limited spaces of the earth's surface, and lines drawn on a globe so as to 

 inclose these spaces, would indicate the "geographical distribution" of 

 these animals. 



There are hardly any s]>ecies of animals and plants which are not in 

 like manner confined within limited geographical areas, and hence if we 

 were to set out from England, and travel either duo south or due north, 



