ISOLATION 183 



but increase their numbers by self -division. This 

 insures that each individual is isolated from alt 

 others, and consequently any variation that may 

 arise will be preserved, unless it is harmful to the 

 individual or is counteracted by reversion. To this, 

 no doubt, we owe the original separation of plants 

 from animals ; as the variations which gave rise to- 

 this separation were useful, they would be taken in 

 hand by natural selection, and so would become 

 progressive. We may also find in asexual repro- 

 duction the cause of the immense variety seen among 

 Diatoms, Fungi, Radiolarians, and Foraminifera. 

 These variations, some of which are the very earliest 

 recorded in the rocks, were non-utilitarian and never 

 progressed far, probably because further progress in 

 any direction would have been harmful. 



Self-fertilisation is nearly as efficient a cause, but 

 a cross may occasionally occur. Ferns, and many 

 other plants, as well as some animals, are thus iso- 

 lated and are able to preserve indifferent variations. 

 A few years ago Count Berg Sagnitz made a large 

 number of observations, with the express purpose 

 of testing the truth of this deduction ; and he found 

 that self-fertilised plants really have more constant 

 varieties than species which are cross-fertilised. 



The prepotency of some pollen grains over others 

 in the fertilisation of flowers is another cause of 

 physiological isolation. And in animals, when an 

 individual of either sex shews great power in trans- 

 mitting its special characters, it also is said to be 

 prepotent, although the two cases are not quite alike. 

 This prepotency in domesticated animals is usually 



