PROBLEMS OF MODERN SCIENCE 



One of the great achievements of evolution, both 

 in plants and animals, has been the enclosure 

 and protection of the young developing organism. 

 Among primitive organisms, such as Alga? and many 

 groups of invertebrate and even vertebrate animals, 

 the reproductive cells are quite commonly dis- 

 charged into the sea-water, where fertilisation 

 occurs, and following this, the whole development 

 takes place in an unprotected condition. Environ- 

 mental fluctuations can do their worst in directly 

 distorting or inhibiting the development of the 

 young. The organism in the most immature and 

 sensitive stages of its life-cycle is completely at the 

 mercy of the elements, and it is not surprising that 

 a very small percentage survive. 



But the higher plants and animals show many 

 steps in the development of special structures 

 and conditions for the protection of their offspring 

 during these early stages. The problem has been 

 more or less successfully solved in a variety of 

 ways. The great advantage of such devices to the 

 species is a sufficient reason for their perpetua- 

 tion and improvement as well as for the various 

 stages of their production. In plants this has 

 resulted in the seed, a structure which has 

 apparently evolved in its main features indepen- 

 dently in different phyla. The last step in the 

 perfection of this process in animals is found in the 

 mammals, in which practically the whole develop- 

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