VARIATIONS IN PARTHENOGENETIC FORMS 61 



marine forms are found from between tide-marks to the 

 deepest abysses of the ocean. Some of the species are 

 among the commonest objects of the seashore, and others 

 may be obtained in numbers by the use of the dredge or 

 trawl. They often occur as delicate encrustations, usually 

 calcareous, on plants, stones, or shells ; or they may assume 

 the appearance of seaweeds, corals, or hydroids. Although 

 most of them are of comparatively small size, they are 

 usually large enough to be recognised by the naked eye, 

 while the largest of them reach a diameter of a foot or two." 



" The statement has often been made, as the result of a 

 theoretical conception of the physical basis of heredity, that 

 the asexual method of reproduction gives rise to little or no 

 variability. Although there are many reasons for doubting 

 the validity of this conclusion, it may be well to state at the 

 outset that the Polyzoa, which are without exception charac- 

 terised by increasing in an asexual manner, show a high 

 degree of variability in the individuals thus produced. So 

 much is this the case, that the want of fixity of type which 

 results from the tendency to vary, renders the definition of 

 species particularly difficult in this group of animals." l 



As an illustration of how easily considerable differences 

 between individuals may escape the observation of the un- 

 trained eye, the case of the white man living among negroes 

 may be considered. At first all the negroes appear alike to 

 the white stranger; he sees little or no difference between 

 them, and is unable to distinguish between the features of 

 individuals belonging to different tribes. The first step in 

 his training will be, that he will be able to distinguish with 

 ease differences between members of different tribes. If he 

 live long enough among them, he will see just as much 

 difference between negro brothers and other near relations 

 as he does between white men. 2 The observation of differences 



1 Harmer, Sidney F., Presidential Address to the Zoological Section, British 

 Association Meeting, 1908. 



2 This was the personal experience of my brother, Major H. A. Walker, after 

 seven years' service with the King's African Rifles. 



