The primary basis for classifying orders in the taxonomic hierarchy is aggregates of characters. Taxonomy is the branch of biology that deals with the classification and naming of organisms. It aims to establish a hierarchical system that reflects the evolutionary relationships among different organisms.

1. Taxonomic Hierarchy:
The taxonomic hierarchy consists of several levels, including kingdom, phylum, class, order, family, genus, and species. Orders are one of the intermediate levels in this hierarchy, between classes and families. They group together closely related families based on shared characteristics.

2. Aggregates of Characters:
The classification of orders is primarily based on aggregates of characters. This means that orders are defined based on a combination of various characteristics exhibited by the organisms within them. These characteristics can be both morphological (related to the structure and form of an organism) and physiological (related to the functions and processes of an organism).

3. Morphological Characteristics:
Morphological characteristics include external features such as the shape, size, color, and texture of an organism, as well as internal structures like organs, tissues, and cells. These features can provide valuable information about an organism's evolutionary relationships and help determine its taxonomic classification.

4. Physiological Characteristics:
Physiological characteristics refer to the functions and processes of an organism, including metabolic activities, reproductive strategies, and ecological adaptations. These characteristics can provide insights into an organism's evolutionary history and its place within the taxonomic hierarchy.

5. Comparing and Contrasting:
By comparing and contrasting the morphological and physiological characteristics of different organisms, taxonomists can identify shared traits that indicate a closer evolutionary relationship. These shared traits are used to group organisms into orders and further refine the taxonomic classification.

6. Importance of Aggregates of Characters:
Using aggregates of characters for classifying orders ensures that organisms with similar characteristics are grouped together, reflecting their shared evolutionary history. It also helps in organizing and understanding the vast diversity of life on Earth.

In conclusion, the primary basis for classifying orders in the taxonomic hierarchy is aggregates of characters. By considering a combination of morphological and physiological characteristics, taxonomists can determine the evolutionary relationships among different organisms and group them into orders accordingly.