212 THE SURVIVAL OF THE UNLIKE. [iX. 



seems to point more strongly in one direction than 

 another. In oiie decade we may think lime to be an 

 indispensable fertilizer, and* in the next it may be 

 discarded ; yet we may eventually find that both posi- 

 tions are tenable. Yet there has been a decided up- 

 uplift in methods of simple tillage and preparation of 

 land and the science of fertilizing the soil ; and, more- 

 over, the application of this knowledge is widespread 

 where it was once local or rare. And the application 

 of machinery and mechanical devices to almost every 

 horticultural labor cannot have escaped the attention 

 of the most careless observer. 



Among specific horticultural industries, the recent 

 evolution of the glass house has been remarkable. In 

 1806 the greenhouse was still a place in which to keep 

 plants green, and M'Mahon felt obliged to disapprove 

 of living rooms over it to keep the roof from freezing, 

 because they are "not only an additional and unneces- 

 sary expense, but they give the building a heavy ap- 

 pearance." The first American greenhouse of which 

 we have a picture, .with a wooden roof and heavy sides, 

 was built in 1764. Glass houses increased in numbers 

 very slowly until the middle of this century, and they 

 can only now be said to be popular. Twenty years 

 ago a glass house was a luxury or an enterprise suited 

 only to large concerns, and the management of it 

 was to most intelligent people an impenetrable mys- 

 tery. At the present time, even the humblest gar- 

 dener, if he is thrifty, can afford a greenhouse. In 

 fact, the glass house is rapidly coming to be an in- 

 dispensable adjunct to nearly all kinds of progressive 

 gardening. The secret of this increasing popularity 

 of the glass house is the simplicity of construction 



