158 THE DARWINIAN THEORY 



as an objection to the Darwinian theory, on the 

 ground that, as the fittest survive, there should be a 

 continual improvement in the race ; whereas, we 

 have seen forms such as Lingula and Nautilus 

 persisting apparently unchanged for periods of 

 enormous duration ; or even through the whole time 

 of which we have any evidence in regard to life on 

 the earth. We must bear in mind, however, that 

 natural selection does not necessarily imply advance, 

 and is perfectly consistent with a stationary con- 

 dition ; for the fittest now may be the fittest many 

 years or ages afterwards. It is quite practicable for 

 particular forms to remain stationary for enormously 

 long periods, provided their environment, or at least 

 all features of the environment affecting them, 

 remain constant. 



Persistent types are usually marine, for there the 

 conditions are more constant. They are very com- 

 monly found on sandy shores, in which they often 

 burrow ; their tenacity of life and power of with- 

 standing injury are very great ; they are usually also 

 unpalatable, and hence not eaten as food by other 

 animals. Their tenacity of life is well shown in the 

 case of Lingula, which was carried by Morse in his 

 pocket during a three days' journey across America 

 without coming to any harm ; and Balanoglossus has 

 been known to live in a bucket of unaerated water 

 in a hot climate, with the hinder part of its body 

 completely macerated, and its branchial skeleton 

 exposed. 



The persistence of lowly organised forms along- 

 side more highly organised ones is often felt as a 



