Introdttctton to Aid mat Morphology. 39 



species, as we are yet imperfectly acquainted with the 

 palaeontological record of past life. Qn the other 

 hand, the necessity for the union of several so-called 

 species (by the discovery of a perfect series of links) 

 has often occurred in the experience of zoologists.* 



In the characters of types, arrangements apparently 

 unconnected with each other always occur in groups : 

 thus, animals that suckle their young have always two 

 occipital condyles and non-nucleated blood corpuscles, 

 and animals clothed with feathers have a muscular 

 right auriculo-ventricular valve. These coincidences 

 are called correlations of growth, and can only be in- 

 telligently accounted for on the ground of a community 

 of descent. 



In tracing the embryonic development of individuals, 

 a retrogression is sometimes noticed instead of a pro- 

 gression. This is the reverse of specialization, and 

 produces a simplification of organs. An animal in a 

 position in which certain organs would be useless, 

 loses them, and thus becomes reduced. Thus, re- 

 duction is the consequence of loss of function. It is a 

 gradual process, organs becoming rudimental before 

 they become finally aborted. These rudiments are 

 called by Gegenbmir, significant finger-posts in compa- 

 rative anatomy. Two of the commonest conditions 

 under which reduction takes place are parasitism and 

 the grouping of personse into colonies. 



A parasite is an animal living on or in the body of 

 another, who is called its host. There are two con- 

 ditions often confounded under the name — ist. Com- 



* Wlien several usually distinct forms are included in one species, each 

 is called a sub-species ; less distinct forms are varieties. As a rule, the 

 groups poorest in species are the most aberrant, 



