THE CROSS-BREEDING OF RACES 143 



changes of embryonic growth ; the other is to contribute 

 some living material from the male parent to the new 

 individual arising from the growth and shaping of the 

 egg-cell. The first influence can be exercised without 

 the second, as is seen in the case of the eggs of some 

 sea-urchins stimulated to growth by the spermatozoa of 

 some star-fishes. It happens that these marine animals 

 are convenient for study and experiment because their 

 eggs are small and transparent and that they and the 

 spermatozoa are freely passed into the sea-water at the 

 breeding season, in which the fertilization of the eggs 

 takes place. 



When these facts are considered we have to admit that 

 in the mating of two species which will not regularly and 

 naturally breed together, there may be a limited action of 

 the spermatic element which may stimulate the egg to 

 development without contributing by fusion in the regular 

 way to the actual substance of the young so produced, or 

 only contributing an amount insufficient to produce a full 

 and normal development of the hybrid young. Such 

 cases not improbably sometimes occur in higher animals, 

 though they have not been, as yet, shown to exist except 

 in the experiments with sea-urchins' eggs and feather- 

 star's sperm. 



In all animals and plants, but especially in domesti- 

 cated and cultivated stocks or strains, varieties arise which, 

 by natural or artificial separation, breed apart, and give 

 rise to what are called "races." Such races in natural 

 conditions may become species. Species are races or 

 groups of individuals, which, by long estrangement (not 

 necessarily local isolation) from the parent stock and by 

 adaptation to special conditions of life, have become more 

 or less " stable " — that is, permanent and unchanging in 

 the conditions to which they have become adapted. They 

 acquire by one device or another the habit of not breeding 



