Causes of Variation 137 



variation is due both to inherited and to outside causes. 

 For from a batch of eggs laid by the same mother and 

 subject to the same conditions, young varying in many 

 respects can be reared ; while it has been proved that 

 by subjecting developing insects to certain derinite 

 surroundings, definite variation in certain directions 

 can be induced. 



As to the cause of hereditary tendencies to vary, we 

 can only speculate. The suggestion has been made 

 that all variations due to inherited influences — con- 

 genital variations, as they are termed — are the result 

 of sexual reproduction (57)- ■^ moment's thought 

 brings home to one the innumerable multitude of 

 ancestors to which each living creature must owe his 

 being ; and the qualities of these ancestors are be- 

 lieved to be passed on in varying proportions to 

 different individual descendants. A question of fact, 

 of the highest interest, which still awaits solution, is 

 whether congenital variation runs in definite directions 

 or whether animals vary in all directions indefinitely. 

 In all probability it will be found that while some 

 characters vary along well-defined lines, others in 

 their variation obey " the law of chance." The 

 amount of variation that may occur is another ques- 

 tion of importance. In some insects a continuous series 

 of varieties showing only slight, almost imperceptible 

 differences from each other can be traced. In others, 

 individuals showing considerable modification seem to 

 arise suddenly from normal parents ; such variation is 

 said to be " discontinuous." For example, an insect 

 whose foot has usually five segments may give rise to 

 variety with a four-segmented foot. The measure- 

 ment of nearly 600 male earwigs has shown the length 

 of the tail-forceps (fig. 84) in those insects to vary 

 from 2-5 mm. to 9 mm. ; but the majority of speci- 

 mens did not, as might have been expected, approach 



