60 CARL H. EIGENMANN. 



scarcely be imagined, and one is lead to suspect that some of the 

 forms are over-adapted. In their divergence in form they have 

 reached almost every conceivable shape as we shall see in a 

 moment. 



The struggle for existence with any unit of physical environ- 

 ment, whether there be geometric rate of increase or not, tends 

 to convergence in habit and form. There is no more striking 

 instance of this than the acceptance of the annual, or decidu- 

 ous, habit of most of the plants inhabiting the temperate zones 

 with their seasonal changes. Records of the simultaneous and 

 similar changes in the form in the mass of species of any area 

 during changing physical conditions are not wanting. For in- 

 stance, Scott says : l 



"The steps of modernization which may be observed in fol- 

 lowing out the history of many different groups of mammals are 

 seen to keep curiously parallel, as may be noticed, for example, 

 in the series of skulls figured by Kowalevsky, where we find 

 similar changes occuring in such families as the pigs, deer, ante- 

 lopes, horses, elephants, etc. Indeed, one may speak with prop- 

 riety of a Puerco, or Wasatch, or White River type of skull, 

 which will be found exemplified in widely separate orders." 



On some riffles of the. San Juan river of Cuba I found a small 

 fish that is very strikingly like other fishes inhabiting similar lo- 

 calities in the eastern United States. The former is a goby, a 

 marine form, Pliilypnus dorniitator, that has become adjusted to 

 the conditions found about the riffles of streams ; the others are 

 darters, Hadropfems, belonging to an entirely different family of 

 fresh-water fishes. The similarity of various " darters " which 

 live on the bottom of our streams to various gobies and blennies 

 that occupy a similar position along marine shores has repeat- 

 edly been noticed. 



In the tropics live many burrowing lizards and snakes. 

 Rliiucnra, one of the lizards, lives and acts like an earthworm 

 and so like an earthworm has it become that only a close inspec- 

 tion reveals its true nature. Even the chickens following the 

 plows in Florida and Cuba are said to be taken in by the simi- 

 larity of some of the burrowing lizards to earthworms. 



\Journ. Morpho!., Y., 365, 1891. 



