504 NATURAL SCIENCE. s^vr.. 



any more than it follows that all cells of the body are alike because 

 all have arisen from one egg. There may be many kinds of ova 

 produced, and the development of these into insects of at least six 

 different kinds may possibly be due to some initial difference in the 

 ova. The same is true of spermatozoa {mutatis mutandis). 



Again, the ova may be fertilised at different stages of maturation. 

 There is a difference between a mature and an immature ovum, and 

 there are stages in maturity. The ovum is a living cell, and its con- 

 stitution, chemical and physical, may vary with its age. The age at 

 which it is fertilised may, therefore, have something to do with its 

 future development. So also with the spermatozoon, and some have 

 held that differences of this kind determine the sex of the offspring 

 in various animals, and especially in mammals. 



If, however, two females are exactly alike in every detail — 

 physiologically as well as structurally — then, however many kinds of 

 ova A produces, B will of necessity produce kinds of ova exactly like 

 them : and a similar likeness will also exist between the spermatozoa 

 of two males, M and N, however many kinds there may be of them. 

 If two like ova, derived from A and B respectively, be fertilised by 

 two like spermatozoa, derived from M and N respectively, then the 

 two fertihsed eggs so produced will be alike in every minutest detail, 

 physically and chemically, and, therefore, also physiologically. 



Of the influences brought to bear upon the fertilised and 

 developing egg, the most important would seem to be temperature, 

 supply of oxygen and of moisture, and quantity and quaHty of food. 

 We know that the quantity and quality of food supplied to the larva 

 developing from the fertilised egg of a hive bee determine whether 

 that larva shall become a queen or a worker. We do not know this 

 of the termite larva, but it is none the less safe to say that if every 

 influence brought to bear upon each of the two like eggs just 

 spoken of be precisely the same in the two cases then those two like 

 eggs will undergo hke changes. Whatever the form and structure of 

 the full-grown insect developing from one of those eggs, that 

 developing from the other must of necessity be like it. The causes 

 are ex hypothcsi exactly alike, the effects must of necessity be alike 

 also. If one of these two insects be a soldier termite, so must the 

 other be. If one be a butterfly, so must the other be. 



The likeness of the influences determining not only the production, 

 out also the formation, the determination of every detail of structure, 

 in any two cases, sufficiently explains the likeness of the two 

 individuals. And this is true even when the two individuals are 

 only distantly related to each other, and when neither of them bears 

 any close resemblance either to its parents, or to any of its ancestors. 



A particular soldier has no eyes and no wings, because the 

 influences which determined the structure of that particular 

 soldier were not such as to lead to the production of either 

 wings or eyes. A second soldier is also without eyes and 



