8 INTRODUCTION [cH. 



occurs in all animals, but the interesting thing about the higher 

 animals is that they give rise to special germs of two kinds, called 

 ova (eggs) and spermatozoa respectively, which cannot develop 

 without first conjugating, one of the first kind uniting with one of 

 the second. The lowest animals also produce such germs in many 

 cases, but sometimes they are all alike and at other times they are 

 different in size. Sometimes indeed they are almost indistinguishable 

 from true ova and spermatozoa. The name gamete is employed 

 therefore to designate conjugating germ cells irrespective of their 

 size. 



The ovum is devoid of the power of movement and has a larger 

 or smaller amount of undigested or at any rate unassimilated food 

 stored in it; this reserve material is called yolk. The spermato- 

 zoon, on the other hand, has no such reserve and is in consequence 

 very much smaller than the ovum, but it possesses in nearly every 

 case the power of movement by which it is enabled to seek and find 

 the ovum. Reproduction, which thus requires conjugation before 

 development can take place is called sexual reproduction. In 

 most cases ova and spermatozoa are developed in different individuals. 

 The individual giving rise to ova is called the female, that giving 

 rise to spermatozoa is termed the male. In this case the species of 

 animals is said to be bisexual. When both ova and spermatozoa 

 are developed in the same individual it is spoken of as her- 

 maphrodite. 



It is obvious to the most casual observation that there is an 

 amazing variety of animals in the world. Closer 

 observation reveals the fact that while no two 

 animals are exactly alike, all can be nevertheless sorted into a 

 number of kinds called species, the individuals composing which 

 apart from the difference between males and females and difference 

 due to age resemble each other exceedingly closely. Where the 

 observation has been made, it is always found that the members 

 of a species conjugate freely with one another ; and indeed this is 

 assumed to be the case in every species ; that is, we group a number 

 of specimens into a species under the assumption that they can 

 conjugate with one another, and that young like themselves will 

 develop as the result. If this can be shown to be not the case, we 

 conclude that a mistake has been made and that two or more species 

 have been confounded with one another. It follows that the vast 

 majority of species rest on provisional hypotheses ; these hypotheses 

 nevertheless possess a very high degree of probability, for by the 



