24 MANUAL OF ZOOLOGY. 



Zoophytes and the Polyzoa are singularly like one another 

 so much so, that they have often been classed together ; 

 whereas, in reality, they belong to different sub -kingdoms. 

 Many other cases of this resemblance of different animals 

 might be adduced, 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 different 

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

 South America is taken by the Euphorbiae of Africa; or, to 

 take a zoological illustration, many of the different orders of 

 Mammalia are represented in the single order Marsupialia in 

 Australia, in which country this order has almost alone to dis- 

 charge the functions elsewhere performed by several orders. 

 Many homomorphous forms, however, live peacefully side by 

 side, and it is difficult to say whether in this case the resem- 

 blance between them is for the advantage or for the disadvan- 

 tage of either. In other cases we find certain animals putting 

 on the external characters of certain other animals, to which 

 they may be closely related, or from which they may be widely 

 separated in zoological position. Such cases are said to be 

 examples of "mimicry," and such animals are said to be 

 " mimetic." Excellent examples of this may be found amongst 

 certain Butterflies, or in the close resemblance of the clear- 

 winged Moths to Bees and Hornets. In all these cases it 

 appears that the mimetic species is protected from some 

 enemy by its outward similarity to the form which it mimics. 

 Finally, there are numerous cases in which animals resemble 

 certain natural objects, and thus greatly diminish their chances 

 of being detected by their natural foes. Excellent instances 

 of this " protective resemblance " are afforded by the insects 

 known as Walking-leaves (Phy Ilium, fig. 2) and Walking-sticks 

 (Phasmid(z\ which respectively present the most singular 

 resemblance to leaves and dried twigs. The student, how- 

 ever, must carefully guard himself against supposing that the 

 term " mimicry " implies any conscious action on the part of 

 the mimetic species ; there being no evidence to support such 

 a view. 



9. CORRELATION OF GROWTH. 



This term is employed by zoologists to express the empirical 

 law, that certain structures, not necessarily or usually connected 

 together by any visible link, invariably occur in association 

 with one another, and never occur apart so far, at any rate, 

 as human observation goes. 



