ipo Changing Plant and Animal Nature 



disadvantage of being a parasite. Generally speaking, the 

 emergence, spread and extinction of species go forward with 

 no relation to our own preconceptions of goodness or beauty. 

 While in general progressive specialization facilitates life 

 and makes possible the intrusion of living things into the gaps 

 left by other species, it carries with it the danger of ex- 

 termination when the specialized conditions are modified, as 

 sooner or later they must be. We find accordingly that there 

 has been in the course of evolution not only such progres- 

 sive specialization with the passing of time, but also a per- 

 sistence through the ages of rather primitive types that have 

 survived all the changes. The horseshoe crab is often referred 

 to as a living fossil because it is the sole survivor of the time 

 when its nearest relative (the trilobites, see page 30) were 

 living in vast numbers and in a great variety of forms. Spe- 

 cies of protozoa have continued from earliest times with very 

 slight changes in type, and indeed some of the present-day 

 forms are quite indistinguishable from their ancestors in the 

 rocks. There are, however, records of highly developed in- 

 sects that have also persisted through millions of years. 

 Among the best preserved fossils are lumps of amber, the 

 hardened resin of trees of ancient times. In these we find 

 buried entire bodies of insects of these former periods, and 

 in comparison with them certain modern species show no 

 changes through all these years. 



Isolation 



In the Sandwich Islands, which are of volcanic origin, 

 the sides of the central peaks are furrowed into relatively 

 deep valleys by the mountain streams. In some of these 

 islands it has been observed that each valley maintains a dis- 

 tinct species of snail. Yet all these snails are so much alike 

 that they are easily conceived to have had a common an- 

 cestry. There are recognized nearly a thousand distinct 

 varieties of which some two hundred are considered good 

 species. In recent years Professor Crampton has made an 

 intensive study of land snails from Tahiti, and finds all the 



