List and explain several major sources of the new wave of imperialism that occurred in the second half of the nineteenth century. To what extent did these ideas find support among the populations of imperialist states?

Whereas early European exploration and colonization was born out of dire necessity (to avoid conquest and enslavement by the invading Ottoman imperialists), European colonization in the second half of the nineteenth century had acquired a variety of ideological and practical justifications.
Wasn't the success of imperialism self-evident? The settlers of the first English colonies in North America had achieved the world's highest living standard by the eighteenth century and by the end of the nineteenth century had created the world's greatest modern industrial power. The second British Empire in India had made the British East India Company one of the world's most successful.
The influence of Charles Darwin's theory of evolution through natural selection of the fittest was applied by Herbert Spencer to human society as Social Darwinism. The Social Darwinists saw European domination as a natural outcome of their own fitness, much as the previously dominant Asian Empires of the Ottomans, Moghuls, and Chinese had attributed their success to their innate superiority and looked down on Europeans as inferior.
Practical justifications included the need for raw materials for industrial production that were unavailable domestically like rubber and rare minerals. Often these resources were ignored or underdeveloped by the natives and the advanced industrial economies could use them in manufactures to raise domestic and global living standards. Indeed, industrial output and global trade was having a profound positive impact on raising global living standards for those willing to participate in the new world economy.
The mission to civilize the world appealed to social reformers and practical men alike. Establishing new markets often required introducing more primitive societies to the self-evident benefits of the Western work ethic, competition, science, medicine, private property rights, law, and Western modes of consumption. Indeed many felt that withholding these benefits in the first world would be a moral crime. The civilizing mission of colonization and imperialism would help to eradicate the twin relics of barbarism (so-called) of polygamy and slavery. What Charles Dickens called "telescopic philanthropy" was a major motivation to provide help to the needy of what would come to be called the third world. Christian missionaries often saw themselves as playing a dual role in spreading the Gospel and the benefits of Western civilization.
There is little doubt that many of these ideas found enthusiastic support among the populations of the imperialist states. The evidence is still found in British material culture from the period where the Empire is an obvious source of national pride. The benefits of imperialism were widely believed to outweigh the costs and, in many cases, the humanitarian motivations were entirely sincere. Europeans and Americans would leave secure lives of prosperity to bring the blessings of civilization, as they saw it, to remote and dangerous parts of the globe often at their own expense and to no reward (sometimes to be killed).
The desire for natural resources was just as real and often the natives benefited from the resulting wealth, some of which was re-invested in the building up of the local infrastructure including modern transport, communication, and education systems. Of course, there were appalling abuses at times like the Belgian exploitation of the Congo, and the popular view today is that the Europeans derived all the benefits, but this was not the popular view at the time. Imperial powers often understood themselves as maternal guardians and protectors of their colonial subjects, and the investments in native education and infrastructure show that the advancement of the natives was thought to be to everyone's benefit.