2 The Unity of the Organism 



furnish perhaps the largest mass of relevant facts we pos- 

 sess from any one field. 



Innumerable researches on fully formed spennatozoa, the 

 greatest single research being that of Retzius, give us ex- 

 tensive knowledge of the variety of structure of the sperma- 

 tozoa in the larger and smaller taxonomic subdivisions of 

 the animal world. It would be going beyond the evidence 

 to say that every well-characterized animal species may be 

 identified by its speraiatozoa ; but unquestionably the trend 

 of investigation has been toward such a conclusion. I 

 believe, for example, it would be impossible to assert on the 

 basis of evidence that any two species of animals belonging 

 to different genera, no matter how much alike if their dis- 

 tinctiveness is not questioned, have indistinguishable sper- 

 matozoa. "One may say," writes Ballowitz, "that each ani- 

 mal species has its own sperm-form of definite size." ^ An 

 attempt to illustrate fully this variety of form and size by 

 specific examples is out of the question here. We will refer 

 only to the specificity of the sperm of man himself. Ret- 

 zius was able to compare in detail sperms of the Chimpanzee, 

 Orang-Utan, Gibbon, and Homo, and found that while their 

 resemblance is rather close, each possesses clear differential 

 marks. For example, the spiral structure of the envelop of 

 the central piece is considerably more distinct in the Chim- 

 panzee than in Homo. Worthy of mention is the fact that, 

 according to Retzius, the sperm of the Chimpanzee resembles 

 that of Homo more closely than does that of the Orang, 

 thus falling in with the fact that in several particulars of 

 adult structure the resemblance of the Chimpanzee to man 

 is closer than that of the Orang. 



The spermatozoa of a given animal group having a closer 

 resemblance to one another than to those of other groups; 

 in other words, having a resemblance due to descent, are 

 themselves subject to heredity and are not alone concerned 

 in the transmission of hereditary attributes from parent to 



