NON-ADAPTIVE CHARACTERS 419 



isolation that has been employed to prevent the swamping 

 effects of intercrossing ; but it is an isolation that may be 

 broken through at any moment, and if all the different varieties 

 of some domestic plant or animal were turned loose to struggle 

 for existence and interbreed with one another and with the 

 parent species in a state of nature, they would probably in most 

 cases very soon cease to have any separate existence. 



The theory of natural selection, combined with that of the 

 gradual inheritance of the effects of use and disuse and of other 

 modifications brought about by the long-continued influence of 

 the environment, affords a satisfactory explanation of the evolu- 

 tion of adaptive characters. Many if not all organisms, how- 

 ever, exhibit characters to which we can assign no adaptive value, 

 which do not seem to be of any particular use to the organism in 

 the struggle for existence, or which apparently might, so far as 

 utility is concerned, be equally well replaced by any one of a 

 number of alternative characters. 



Amongst the microscopic Protozoa species are frequently 

 distinguished from one another by minute differences in the form 

 or ornamentation of the skeleton (compare Figs. 3 and 4). 

 Different species of the genus Lagena (Fig. 4), 1 amongst the 

 Foraminifera, for example, exhibit different sculptured patterns 

 upon their flask-shaped calcareous shells. Are we to suppose 

 that it is of any consequence to the gelatinous, Amoeba-like 

 inhabitant of the flask whether its shell be ornamented in one 

 way rather than in another ? Does one pattern help a uni- 

 cellular foraminiferan or radiolarian more Lhan another in the 

 struggle for existence ? The same argument applies to the 

 extremely minute siliceous flesh-spicules or microscleres 

 which occur scattered without order through the ground sub- 

 stance of many sponges, and the form of which is regarded by 

 those who have studied the question as by far the most reliable 

 guide, not only in the recognition of species, but also in the 

 grouping of these species in genera and families. Take, for 

 example, the wonderful chelae (Fig. 187), characteristic of the 

 family Desmacidonidae. Different genera and species are dis- 

 tinguished by differences in the size, shape and number of 



1 The two figures below the centre figure and two in the bottom right hand 

 corner represent four species of Lagena. 



E E 2 



