Generation of Animals

Final Cause of Spermatic Organs

With regard to the difference of the spermatic organs in males, if we are to investigate the causes of their existence, we must first grasp the final cause of the testes. Now if nature makes everything either because it is necessary or because it is better so, this part also must be for one of these two reasons. But that it is not necessary for generation is plain because neither serpents have testes nor have fish... It remains then that it must be because it is somehow better so... [Those animals] which have not testes but only ducts... are all quicker in accomplishing copulation. But those which are to be more temperate... have their ducts twisted to prevent their desire being too violent and hasty. It is for this that the testes are contrived; for they make the movement of the spermatic secretion steadier....

Serpents copulate twining round one another, and ... have neither testes nor penis, the latter because they have no legs, the former because of their length, but they have ducts like fish; for on account of their extreme length the seminal fluid would take too long in its passage and be cooled if it were further delayed by testes. (This happens also if the penis is large; such men are less fertile than when it is smaller because semen, if cold, is not generative, and that which is carried too far is cooled). So much the reason why some animals have testes and others not.

- Aristotle, "Generation of Animals" (4th cent. BCE)

Discussion: note the repeated use of the word "because" as literally meaning "by cause of". Are Aristotle's observations accurate? How might a modern evolutionary biologist explain the occurrence of testes in serpents?