124 TRANSACTIONS AND PROCEEDINGS OF THE [Skss. lxiii. 



4. The Zoospore. — A free swimming organism, which 



moves by means of cilia. This also was evidently 

 of polar origin. The zoospore floated or swam 

 away from the dark pole, and, after reaching 

 the eqnator, and it may be the other pole, became 

 attached and entered on algoid life. 



5. Isogamy. — Union of two garnets. 



6. Sex. 



It is probable that sex did not make its appearance till 

 comparatively late in the world's history, and this for two 

 reasons — (1) because many land plants show no trace of it, 

 so that it probably did not appear among algie till after 

 land plants left the ocean for the land ; (2) by considering 

 what is the use of sex in the economy of an organism. 



A glance at the facts will show that the use of sex is to 

 check variation. 



When a sport occurs, as in the case of the well-known 

 copper beech, that sport is reproduced by budding ; but if 

 the plant were to seed, the seedling would return to the 

 old type. This law is supported by the universal experi- 

 ence of all gardeners. One parent being typical, and the 

 other abnormal, tlie seedling naturally tends to return to 

 the old or well-established type. 



The observations of C. Hurst, on " Hybridisation in 

 Orchids," has thrown considerable light on the nature of 

 sex. He shows that in the case of hybrids among orchids, 

 it is the pollen of the hybrid that is impaired, and that 

 the seed can generally be fertilised by the pollen of either 

 parent. On the contrary, the pollen of the hybrid cannot 

 fertilise a hybrid seed, or a seed of either of the parent 

 plants. This is very like what liappens in the case of 

 mules between the ass and mare. The female mule 

 occasionally breeds with a liorse or ass. The male mule 

 cannot breed with u mare, a slie ass, or a female mule. 



Down to the time of Darwin, a species was defined as 

 a group of individual plants, so constituted that the pollen 

 of one individual could fertilise the seed of anotlier. 



As this is not the accepted definition of a species at the 

 present day, I must give from the older writers, proofs that 

 it was the accepted definition at the time when Darwin 

 wrote. 



