HOMOMORPHISM. 51 



are said to be " homomorphic." Thus, the Hydroid Zoo- 

 phytes and the Sea-mosses (Polyzod) are singularly like one 

 another in external form ; so much so that they have often 

 been classed together, whereas they differ very greatly in 

 their anatomical characters. Many other instances might 

 be adduced of this close external resemblance between 

 animals and plants which have little or no real relationship 

 with one another ; and in many cases these " representative 

 forms " appear to be able to fill each other's places in the 

 general economy of nature. This is so far true, at any rate, 

 that " homomorphous " forms are generally found in differ- 

 ent parts of the earth's surface. Thus, the place of the 

 Cadi in South America is taken by the Euphorbia of Africa ; 

 or, to take a zoological illustration, many of the different 

 orders of the mammals are represented by the sections of the 

 single order of the Marsupialia. This order, namely, is the 

 almost exclusive possessor of the entire Continent of Aus- 

 tralia, and being thus confronted with very varying condi- 

 tions, and enjoying the almost unlimited freedom of an 

 enormous area, the order has to singly discharge the func- 

 tions which are elsewhere performed by several orders. 

 Homomorphous forms, however, are not universally found 

 in areas widely removed from one another ; and it is very 

 difficult to account for their origin in any case. The older 

 view, advocated by the late Edward Forbes, was that " re- 

 presentative " forms, similar to, but specially distinct from, 

 one another, were created independently in areas which 

 presented similar conditions and environments. The more 

 modern view would regard " homorphous " forms as pro- 

 duced by the action of similar conditions upon organisms 

 primitively not very unlike one another ; so that " homo- 

 morphism " would thus become a form of the "homoplasy" 

 of Mr Lankester. This explanation, however, would still 

 leave much of this subject unexplained. 



IV. MIMICRY. Many instances are known amongst 

 animals in which certain species put on the external charac- 



